Space has three dimensions, and if you do not want to stay at your destination, you can buy a return ticket. Time has but a single dimension, and only one-way travel is allowed: we remember a definite, youthful past but can only imagine possible futures where we grow old and decay. Even though the fundamental laws acting on individual atoms appear to care naught for the direction of time’s axis, macroscopic phenomena most definitely do.
To resolve this conundrum, any physicist will refer you to the second law of thermodynamics, and the concept of increasing entropy. The familiar progression from order to disorder is explained as a consequence of statistics: a game of chance becomes effective certainty when more than a few particles are involved. But is it really that simple? Are all the issues resolved?
The final answers to these questions almost certainly lie in the future, as the subtitle to Sean Carroll’s book From Eternity to Here: The Quest for the Ultimate Theory of Time suggests. However, discussing them in the present can still be an interesting exercise. The ideas that Carroll, a physicist at the California Institute of Technology, puts into play are fascinating in their extent, scope and description. They cover areas as diverse as time in special and general relativity (including the question of time travel), entropy and the arrow of macroscopic time, the psychology of time; and the nature of the beginning and end of time.
Carroll’s main theme is the meaning of time’s arrow as embodied in the second law, and its relationship with cosmology and the origin of the universe. He pays particular attention to the conundrum of time as a one-way trip – an oddity that is all the more perplexing because, Carroll believes, the fundamental laws at a microscopic level are time-reversible. But while this may be true, there are nonetheless hints of time asymmetry in the behaviour of certain fundamental particles. We also inhabit a universe where, apparently, there is a gross asymmetry between matter and antimatter. Until we understand the origin of the matter/antimatter asymmetry – which is critical for our existence – I would hesitate to draw conclusions about the extent of other possible asymmetries.
These are deep waters, but Carroll navigates them successfully, thanks in part to extensive and effective footnotes that enable him to separate more technical remarks from the flow of a readable main text. This is a technique that works well. Were it used more widely in the genre, it could enable physics books to reach a wider readership, by allowing those who want to explore deep ideas to do so without at the same time frightening off more general readers. It did, however, provide me with one of the book’s few minor irritants. Occasionally a footnote was used for some more trite remark, as if the author was embarrassed to have put something meaty in the main text and wanted a jokey aside to sweeten the pill. The comparison between the public’s perceptions of Einstein and Paris Hilton was, I felt, particularly grating.
My other minor quibble concerns physics. Throughout much of the text, entropy is described as if it is a pure number. Yet in the notes and in at least one appearance in the main body, it is described in terms of Boltzmann’s constant, and hence carries dimensions of energy per degree. There was sometimes confusion as to whether entropy or log(W) was being discussed, and to what extent, if any, this mattered. If this was explained, I missed it.
Whatever the precise definition of entropy, if it should turn out that time’s arrow for macroscopic objects is tied to entropic increase, there is an unresolved enigma: why was entropy so small at the Big Bang? This forms one of the more powerful themes in Carroll’s book.
The issue of creation is itself an enigma, though not in the way creationists have argued. Some have claimed – erroneously – that the appearance of life requires a decrease in entropy, and thus implies a violation of the second law. Carroll neatly dismantles such claims by pointing out that if they were true, then refrigerators could not exist. The difference between closed and open systems is critical here, as in so many cases.
This much is well known. What I found intriguing was that Carroll then goes on to examine the entropy problem in a quantitative fashion. The sky, he writes, contains a hot Sun in a cold background – the very epitome of a non-equilibrium situation. For every high-energy photon that arrives here from the Sun, the Earth radiates 20 lower-energy photons into space. This increase in entropy exceeds the local decrease produced by the collective efforts of the biosphere. Nevertheless, if the micro-states of the planet started from utter disorder, the entire biomass could still be converted into a state of high order by such processes. And how long would this require? As far as the second law is concerned, Carroll claims, a year would be enough. Ironically, it seems that the creationists have aimed at the wrong target: physics, far from being inconsistent with biblical accounts of human existence, would allow the entire biomass to emerge within a single year, and certainly within 6000. Over to the biologists as to why it actually took billions!
The real creation of our observable universe, 13.6 billion years ago, suggests an even bigger conundrum: how did the universe’s initial state of low entropy arise? One possibility is that in an infinite and everlasting universe, entropy fluctuates. It is therefore conceivable that we could be in a 14 billion year period in which entropy has increased following a long-ago downward fluctuation, the end of which we perceive to be the start of “time”. Carroll examines this thesis, and points out its flaw: a random fluctuation capable of producing human beings would be remarkable enough (although had it not happened we would not be here to ask the question), so it seems too much to accept an entropic fluctuation that produced the order encoded in galaxies of stars – and much else that, so far as we can tell, is unnecessary for our existence.
Carroll does not discuss whether it might be “easier” to fluctuate billions of galaxies into existence than to produce sentient life. After all, if thermodynamics alone could produce a biosphere in a year, biology must introduce lots of “friction” into evolution. Billions of galaxies courtesy of fluctuation, combined with the chance that there is an Earth-like environment somewhere, might be a more efficient route to a winning lottery ticket. Can we rule that out so easily? Although it is hugely unlikely, is it any less likely than the chance that out of the effectively infinite possible combinations of DNA, it was the ones that made me and you that burst into life, enabling us to know that there is a universe?
Possibly it is. You might disagree with Carroll; you might disagree with me; but a book that makes you think is worth reading. Whether the future will show Carroll’s ideas are forever or just the latest in a never-ending debate, only time will tell.
History is replete with tales of scientists behaving badly, particularly when one of them dares to challenge the theories of another. Faraday battled with Ampère about the finer points of electromagnetic theory. Einstein sharply disagreed with his colleagues about the emerging field of quantum mechanics, famously declaring that “God does not play dice with the universe”. And Newton fought with everybody, including Huygens, Hooke, Flamsteed and, of course, Leibniz.
Readers who do not understand the passionate intensity of scientific arguments may find the events in Juli Zeh’s novel Dark Matter perplexing. But those who do will feel an instant affinity for the book’s central characters Sebastian and Oskar, two physicists who “were said to love physics even more than they loved each other, and [who] fought over it with the passion of rivals”. As their story shows, the opposite of love is not hate; it is indifference. Unfortunately, their shared intellectual love affair takes a destructive turn that reverberates throughout the narrative.
Zeh tells her story in bits and pieces, allowing these to accumulate slowly until they form a dazzling whole. In the beginning, we see two lifelong friends bitterly debating the philosophical implications of a physics theory that invokes the possibility of parallel worlds. Then a young boy is kidnapped and a diabolical ransom demanded. An anaesthesiologist meets a grisly end. A loyal wife loses faith in her husband. A scientist’s carefully structured life unravels. And eventually an unorthodox detective with a love of physics and an inoperable brain tumour steps in to solve his final case by connecting these seemingly random events.
A bestseller in Germany when it first appeared in 2007 under the title Schilf, this new English translation of Dark Matter (published as In Free Fall in the US) follows in the footsteps of other novels by authors who have found inspiration in esoteric physics, notably Jeannette Winterson’s Gut Symmetries and Jonathan Lethem’s As She Crawled Across the Table. But where Winterson embraced string theory and Lethem mined the mother lode of wormholes and extra dimensions, Zeh finds her muse in the “many worlds” interpretation of quantum mechanics.
First proposed in the 1950s by the physicist Hugh Everett III, the premise of the controversial “many worlds” hypothesis is straightforward enough. In any quantum system, every possible outcome for an experiment is present simultaneously in a superposition of states. The sum of all those outcomes is described by the wavefunction. It is only when we observe the system by making a measurement that the wavefunction collapses and all of those possibilities reduce to a single “real” event: the outcome of our observation.
But what happens to those other possibilities once the wavefunction has collapsed? The strictest interpretation of quantum theory simply assumes that by necessity all the other potential outcomes vanish once a measurement is made. Everett offered an alternative: perhaps the wavefunction continues to evolve, forever splitting into other wavefunctions in a never-ending tree, with every branch becoming an entire universe. In this way, every potential outcome contained in the wavefunction – a photon appearing as a particle or wave; a boy being kidnapped or not kidnapped – is realized in its own separate universe. Perhaps, as Sebastian puts it, “Everything that is possible happens.”
In Zeh’s novel, “many worlds” becomes a richly complex metaphor for regret over the road not taken. As in physics, so in life: our choices collapse our wavefunction and set us on a certain course. Sebastian’s wife, Maike, exists in a nebulous superposition of states until one day she meets her future husband on the street and her wavefunction collapses into marriage and motherhood. But perhaps there exists a parallel universe where she made a different choice, with a very different outcome.
Dark Matter is filled with split universes. Inseparable back in their university days, Sebastian quarrels with Oskar out of jealousy, and their personal and professional paths diverge. Yet even though he has chosen a rather sedate, traditional life as a happily married academic, Sebastian is filled with regret at what he has lost: those heady, passionate early days with Oskar, his intellectual soul mate. He clings to the notion of many worlds, reasoning that “[T]here must be other universes in which things went differently… In which Oskar [and I] would never lose each other.”
For his part, Oskar is equally bent on forcing his friend to confront the reality of the break, with an eye toward winning him back. It is a strategy with tragic consequences. But by far the most compelling character is Detective Schilf, whose world diverged into “before” and “after” following the loss of his wife and child. And now his mind is splitting, too, thanks to a brain tumour that he nicknames “the Observer”.
Zeh skilfully pulls together these disparate threads into a compelling intellectual thriller, in which the “villain” turns out to be as mysteriously elusive as the quantum theory of gravity Oskar pursues so single-mindedly. She only stumbles once, with the inexplicable inclusion of a ham-fisted chapter that consists of little more than Sebastian’s monologue detailing his thoughts about time, causality, coincidence, free will and the multiverse. It is overly didactic and jolts the reader out of the story just as the narrative reaches its climax. Zeh’s prose is most effective when she lets her big ideas lurk in the background, rather than take centre stage.
That quibble aside, Dark Matter admirably showcases Zeh’s meticulous plotting, skilful foreshadowing and lyrical turns of phrase; Christine Lo’s translation is sparsely elegant. Perhaps in a different novel, the motives of Zeh’s characters, and their wildly irrational responses to events as they unfold, would strike the reader as highly improbable, straining the willing suspension of disbelief to a breaking point. But in a fictional world where “everything that is possible happens”, these are just other branches in the wavefunction.
Arguably the greatest mystery facing humanity today is the prospect that 75% of the universe is made up of a substance known as “dark energy”, about which we have almost no knowledge at all. Since a further 21% of the universe is made from invisible “dark matter” that can only be detected through its gravitational effects, the ordinary matter and energy making up the Earth, planets and stars is apparently only a tiny part of what exists. These discoveries require a shift in our perception as great as that made after Copernicus’s revelation that the Earth moves around the Sun. Just 25 years ago most scientists believed that the universe could be described by Albert Einstein and Willem de Sitter’s simple and elegant model from 1932 in which gravity is gradually slowing down the expansion of space. But from the mid-1980s a remarkable series of observations was made that did not seem to fit the standard theory, leading some people to suggest that an old and discredited term from Einstein’s general theory of relativity – the “cosmological constant” or “lambda” (Λ) – should be brought back to explain the data.
This constant had originally been introduced by Einstein in 1917 to counteract the attractive pull of gravity, because he believed the universe to be static and eternal. He considered it a property of space itself, but it can also be interpreted as a form of energy that uniformly fills all of space; if Λ is greater than zero, the uniform energy has negative pressure and creates a bizarre, repulsive form of gravity. However, Einstein grew disillusioned with the term and finally abandoned it in 1931 after Edwin Hubble and Milton Humason discovered that the universe is expanding. (Intriguingly, Isaac Newton had considered a linear force behaving like Λ, writing in his Principia of 1687 that it “explained the two principal cases of attraction”.)
Λ resurfaced from time to time, seemingly being brought back into cosmology whenever a problem needed explaining – only to be discarded when more data became available. For many scientists, Λ was simply superfluous and unnatural. Nevertheless, in 1968 Yakov Zel’dovich of Moscow State University convinced the physics community that there was a connection between Λ and the “energy density” of empty space, which arises from the virtual particles that blink in and out of existence in a vacuum. The problem was that the various unrelated contributions to the vacuum energy meant that Λ, if it existed, would be up to 120 orders of magnitude greater than observations suggested. It was thought there must be some mechanism that cancelled Λ exactly to zero.
In 1998, after years of dedicated observations and months of uncertainty, two rival groups of supernova hunters – the High-Z Supernovae Search Team led by Brian Schmidt and the Supernova Cosmology Project (SCP) led by Saul Perlmutter – revealed the astonishing discovery that the expansion of the universe is accelerating. A cosmological constant with a value different to that originally proposed by Einstein for a static universe – rebranded the following year as “dark energy” – was put forward to explain what was driving the expansion, and almost overnight the scientific community accepted a new model of the universe. Undoubtedly, the supernova observations were crucial in changing people’s perspective, but the key to the rapid acceptance of dark energy lies in the decades before.
Inflation and cold dark matter
Our story begins in 1980 when Alan Guth, who was then a postdoc at the Stanford Linear Accelerator Center in California, suggested a bold solution to some of the problems with the standard Big Bang theory of cosmology. He discovered a mechanism that would cause the universe to expand more, in a time interval of about 10–35 s just after the Big Bang, than it has done in the estimated 13.7 billion years since. The implications of this “inflation” were significant.
1 Geometry of the universe In flat, Euclidean space (left), light always travels in straight lines, but in the presence of a gravitational field, space is curved. This causes light rays to bend when passing near to a massive body such as the Sun. If the matter/energy density in the universe is greater than a critical value, then space will be positively curved, like a 3D version of the surface of a sphere, and light rays will converge on each other (middle). But if there is less matter and energy in the universe than the critical density, space will be negatively curved, like a saddle, and light rays will diverge (right).
Einstein’s general theory of relativity, which has so far withstood every test made of it, tells us that the curvature of space is determined by the amount of matter and energy in each volume of that space – and that only for a specific matter/energy density is the geometry Euclidean or “flat” (figure 1). In inflationary cosmology, space is stretched so much that even if the geometry of the observable universe started out far from flat, it would be driven towards flatness – just as a small patch on the surface of a balloon looks increasingly flat as the balloon is blown up. By the mid-1980s a modified version of Guth’s model was overwhelmingly accepted by the physics community.
The problem was that while inflation suggested that the universe should be flat and so be at the critical density, the actual density – calculated by totting up the number of stars in a large region and estimating their masses from their luminosity – was only about 1% of the required value for flatness. In other words, the observed mass density of conventional, “baryonic” material (i.e. protons and neutrons) appeared far too low. Moreover, the amount of baryonic matter in the universe is constrained by the theory of nucleosynthesis, which describes how light elements (hydrogen, helium, deuterium and lithium) formed in the very early universe. The theory can only match observations of the abundances of light elements if the density of baryonic matter is 3–5% of the critical density, with the actual value depending on the rate of expansion.
To make up the shortfall, cosmologists concluded that there has to be a lot of extra, invisible non-baryonic material in the universe. Evidence for this dark matter had been accumulating since 1932, when Jan Oort realized that the stars in the Milky Way are moving too fast to be held within the galaxy if the gravitational pull comes only from the visible matter. (At about the same time, Fritz Zwicky also found evidence for exotic hidden matter within clusters of galaxies.) Inevitably, the idea of dark matter was highly controversial and disputes over its nature rumbled on for the next 50 years. In particular, there were disagreements about how fast the dark-matter particles were moving and how this would affect the formation of large-scale structure, such as galaxies and galaxy clusters.
Then in March 1984, a paper by George Blumenthal, Sandra Faber, Joel Primack and Martin Rees convinced many scientists that the formation of structure in the universe was most likely if dark-matter particles have negligible velocity, i.e. that they are “cold” (Nature 311 517). They found that a universe with about 10 times as much cold dark matter (CDM) as baryonic matter correctly predicted many of the observed properties of galaxies and galaxy clusters. The only problem with this “CDM model” was that the evidence pointed to the total matter density being low – barely 20% of the critical density. However, because of the constraints of inflation, most scientists hoped that the “missing mass density” would be found when measurements of dark matter improved.
Problems with the standard theory
At this point the standard cosmological model was a flat universe with a critical density made up of a small amount of baryonic matter and a majority of CDM. Apart from the fact that most of the matter was thought to be peculiar, this was still the Einstein–De Sitter model, and the theoretical prejudice for it was strong. Unfortunately for the inflation plus CDM model, it came with one very odd prediction: it said that the universe is no more than 10 billion years old, whereas, at the time, some stars were thought to be much older. For this reason, and because observations of the distribution of matter favoured a low mean mass density, the US cosmologists Michael Turner, Gary Steigman and Lawrence Krauss published a paper in June 1984 that investigated the possibility of a relic cosmological constant (Phys. Rev. Lett. 52 2090).
The presence of Λ would cause a slight gravitational repulsion that acts against attractive gravity, meaning that the expansion of the universe would slow down less quickly. This implied that the universe was older than people thought at the time and so could accommodate its most ancient stars. Although Turner, Steigman and Krauss realized that Λ could solve some problems, they were – considering the constant’s chequered past – still wary about including it in any sensible theory. Indeed, the trio paid much more attention to the possibility that the additional mass density required for a flat universe was provided by relativistic particles that have been created recently (in cosmological terms) from the decay of massive particles surviving from the early universe.
One of the other people to tentatively advocate the return of the cosmological constant was James Peebles from Princeton University, who at the time was studying how tiny fluctuations in the density of matter would grow, due to gravitational attraction, then ultimately collapse to form galaxies. Writing in a September 1984 paper in The Astrophysical Journal (284 439), he likewise deduced that the data pointed to an average mass density in the universe of about 20% of the critical value – but he went further and said it might be reasonable to invoke a non-zero cosmological constant in order to meet the new constraints from inflation. Although Peebles was fairly cautious about the idea, this paper helped bring Λ out of obscurity and began to pave the way to the acceptance of dark energy.
Cosmic clues Fluctuations in the cosmic microwave background reveal that the universe is flat but that the mass density is far short of the critical density. (Courtesy: NASA)
It would be tempting to think that the path to dark energy was now clear. However, astronomers realized that the mean mass density of the universe could still be as high as the critical density if there was a lot of extra dark matter hidden in the vast spaces, or voids, between clusters of galaxies. Indeed, when Marc Davis, George Efstathiou, Carlos Frenk and Simon White (following work by Nick Kaiser) ran computer simulations of the evolution of a universe dominated by CDM, they found dark matter and luminous matter were distributed differently, with more CDM in the voids. If galaxies formed only where the overall mean density was high, the simple Einstein–De Sitter flat cosmology could still agree with observations and we would not need to invoke the idea of dark energy at all.
But for those opposed to the idea of dark energy, the problem was that there was no sign of lots of missing mass in the voids. Indeed, when Lev Kofman and Alexei Starobinskii calculated the size of the tiny temperature variations in the cosmic microwave background (CMB) radiation, using different models of the universe, they found that adding Λ to the CDM model predicted fluctuations that would provide a better explanation of the observed distribution of galaxy clusters. Even if Λ were not included in the theory, observations in the late 1980s suggested that cosmological structure on very large scales could be more readily explained by a low-density universe and this, obviously, was incompatible with inflation.
Nevertheless, many people continued to believe that the idea of introducing another parameter, such as Λ, went against the principle of Occam’s razor, given that the data were still so poorly determined. It was not so much that physicists were deeply attached to the standard model, more that, like Einstein, they did not want to complicate the theory unnecessarily. Indeed, at the time, almost anything seemed preferable to the addition of Λ. As George Blumenthal, Avishai Dekel and Primack commented in 1988, introducing Λ would require “a seemingly implausible amount of fine_tuning of the parameters of the theory” (Astrophys. J. 326 539).
They instead proposed that a low-density, negatively curved universe with zero cosmological constant could explain the observed properties of galaxies, even up to large scales, if CDM and baryons contributed comparably to the mass density. This model, they admitted, conflicted with nucleosynthesis bounds, with inflation’s prediction that the universe is flat and with the small observational limit on the size of fluctuations in the CMB, but they believed that there were potential solutions. It seemed so much more aesthetically pleasing for Λ to simply be zero.
Surprising results
The quiet breakthrough came in 1990. Steve Maddox, Will Sutherland, George Efstathiou and Jon Loveday published the results of a study of the spatial distribution of galaxies, based on 185 photographic plates obtained by the UK Schmidt Telescope Unit in Australia (Mon. Not. R. Astron. Soc. 242 43). High-quality, glass copies of the plates were scanned using an automatic plate measuring (APM) machine that had recently been developed at Cambridge University by Edward Kibblewhite and his group. This remarkable survey – the largest in more than 20 years – covered more than 4300 square degrees of the southern sky and included about two million galaxies, looking deep into space and far back in time.
Astonishingly, the results from the APM galaxy survey did not match the standard CDM plus inflation model at all. On angular scales greater than about 3°, the survey provided strong evidence for the existence of galaxy clustering that was simply not predicted by the standard model. In 1990 Efstathiou, Sutherland and Maddox wrote a forthright letter to Nature (348 705), in which they argued that CDM and baryons accounted for only 20% of the critical density. The remaining 80%, they inferred, was provided by a positive cosmological constant, and this soon became known as the ΛCDM model.
The case for a low-density, CDM model came from the APM galaxy survey and a redshift survey of over 2000 galaxies detected by the infrared astronomical satellite (IRAS). The case for a positive cosmological constant now had several arguments in its favour: inflation, which required a flat universe; the small size of the temperature fluctuations in the CMB; and the age problem. “A positive cosmological constant”, wrote Efstathiou, Sutherland and Maddox, “could solve many of the problems of the standard CDM model and should be taken seriously.” This was the strongest appeal yet made in favour of bringing Einstein’s Λ back into cosmology. The APM result for a low mass density of the universe was later confirmed by the 2dF Galaxy Redshift Survey and the Sloan Digital Sky Survey.
Return of the cosmological constant
Soon, others also began looking seriously at the case for ΛCDM. For example, in 1991 one of the present authors (OL), with Per Lilje, Primack and Rees, studied the implications of the cosmological constant on the growth of structure, and concluded it agreed with data available at the time (Mon. Not. R. Astron. Soc. 251 128). But researchers were still reluctant to embrace Λ fully. In 1992 Sean Carroll, William Press and Edwin Turner underlined the problems of considering Λ as the energy density of the vacuum – the coincidence problem (figure 2) and the fact that quantum mechanics predicts a far higher value than observations permit (Ann. Rev. Astron. Astrophys. 30 499). They pointed out that a flat universe plus Λ model effectively required the inclusion of non-baryonic CDM, which meant there were then two highly speculative components of the universe.
2 Density evolution In the Einstein–De Sitter model the universe consists only of components – matter and radiation – that give rise to ordinary, attractive gravity. As the universe expands, gravity slows the expansion and the mass density, ρm (blue), and radiation density, ρrad (red), of the universe decrease, at a rate given by Einstein’s general theory of relativity. In this model there is nothing special about the era in which humans have evolved. If, however, there is a non-zero cosmological constant, it will always keep the same value. So, it seems an unbelievable coincidence that we are living during the brief cosmological era when the dark energy density, ρΛ (green), and the mass density are about the same size. The repulsive gravity of Λ has only recently (in cosmological timescales) begun to dominate and is causing the expansion of the universe to accelerate. Adapted from R A Freedman and W J Kaufmann Universe (2005 W H Freeman)
In 1993 an article appeared in Nature (366 429) on the baryon content of galaxy clusters, written by Simon White, Julio Navarro, August Evrard and Carlos Frenk. They studied the Coma cluster of galaxies, which is about 100 megaparsecs away from the Milky Way and contains more than 1000 galaxies. It is assumed, from satellite evidence, to be typical of all clusters rich in galaxies and its mass has three main components: luminous stars; hot, X-ray emitting gas; and dark matter. White and colleagues realized that the ratio of baryonic to total mass in such a cluster would be fairly representative of the ratio in the universe as a whole. Plugging in the baryon mass fraction from the nucleosynthesis model would then give a measure of the universe’s mean mass density.
After taking an inventory, using the latest data and computer simulations, the group concluded that the baryonic matter was a larger fraction of the total mass of the galaxy cluster than was predicted by a combination of the nucleosynthesis constraint and the standard CDM inflationary model. Baryons could have been produced in the cluster during its formation (by cooling, for example) but the number created would not have been enough to explain the discrepancy. The most plausible explanations were either that the usual interpretation of element abundances (nucleosynthesis theory) was incorrect, or that the mean matter density fell well short of the critical density. Once again, the standard CDM model, with the mass density equal to the critical density, was inadequate. The way to satisfy the constraint from inflation that the universe is flat would be to add a cosmological constant.
Luckily for the theorists, astronomers continued to refine and improve their observations through the development of new equipment and techniques. In particular, tiny variations in the CMB – as measured by NASA’s Cosmic Background Explorer (COBE) satellite and later by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite – dramatically showed that the mass density fell far short of the critical density and yet favoured a universe with an overall flat geometry. Something other than CDM must be providing the mass/energy needed to reach the critical value.
Although there were still a number of proposed variations on the CDM model, ΛCDM was the only model to fit all the data at once. It appeared that matter accounted for 30–40% of the critical density and Λ, as vacuum energy, accounted for 60–70%. Jeremiah Ostriker and Paul Steinhardt succinctly summed up the observational constraints in 1995 in an influential letter to Nature (377 600). The case rested strongly on measurements of the Hubble constant and the age of the universe, and results from the Hipparcos satellite in 1997 finally brought age estimates of the oldest stars down to 10–13 billion years.
Most physicists were still reluctant to consider the idea that Λ should be brought back into cosmology, but the stage was now set for a massive shift in opinion (see “Dark energy” by Robert P Crease). Some people suggested other possibilities for the missing component, and the name “dark energy” was introduced by Michael Turner in 1999 to encompass all the ideas. When the supernova data of the High-Z and SCP teams indicated that the expansion of the universe is accelerating, the rapid embrace of dark energy was in large part due to the people who had argued for its return in the 1980s and 1990s.
Towards a new paradigm shift?
If recent history can teach us anything, it is to not ignore the evidence when it is staring us in the face. While the addition of two poorly understood terms – dark energy and dark matter – to Einstein’s theory may spoil its intrinsic elegance and beauty, simplicity is not in itself a law of nature. Nevertheless, although the case for dark energy has been strengthened over the past dozen years, many people feel unhappy with the current cosmological model. It may be consistent with all the current data but there is no satisfactory explanation in terms of fundamental physics. As a result, a number of alternatives to dark energy have been proposed and it looks likely that there will be another upheaval in our comprehension of the universe in the decade ahead (see “Future paradigm shifts?” below).
3 Evidence for cosmic acceleration Data from type 1a supernovae (blue), baryon acoustic oscillations (green) and the cosmic microwave background (orange) provide independent information on the nature of dark energy. Only a very small region where the amount of matter, Ωm ≈ 0.25, and the amount of dark energy, ΩΛ ≈ 0.75, matches all three data sets. They converge at the value needed for a flat universe: Ωm + ΩΛ = 1. Adapted from M Kowalski et al. 2008 Astrophys. J.686 749
The whole focus of cosmology has altered dramatically and many astronomical observations now being planned or under way are mainly aimed at discovering more about the underlying cause of cosmic acceleration. For example, the ground-based Dark Energy Survey (DES), the European Space Agency’s proposed Euclid space mission, and NASA’s planned space-based Joint Dark Energy Mission (JDEM) will use four complementary techniques – galaxy clustering, baryon acoustic oscillations, weak gravitational lensing and type 1a supernovae – to measure the geometry of the universe and the growth of density perturbations (figure 3). The DES will use a 4 m telescope in Chile with a new camera that will peer deep into the southern sky and will map the distribution of 300 million galaxies over 5000 square degrees (an eighth of the sky) out to a redshift of 2. The five-year survey involves over 100 scientists from the US, the UK, Brazil and Spain, and is due to begin in 2011.
The mystery of dark energy is closely connected to many other puzzles in physics and astronomy, and almost any outcome of these surveys will be interesting. If the data show there is no longer a need for dark energy, it will be a major breakthrough. If, on the other hand, the data point to a new interpretation of dark energy, or to a modification to gravity, it will be revolu_tionary. Above all, it is essential that astrophysics continues to focus on a diversity of issues so that individuals have the chance to do creative research and suggest new ideas. The next paradigm shift in our understanding may not come from the direction we expect.
Future paradigm shifts?
Cosmologists still have no real idea what dark energy is and it may not even be the answer to what makes up the bulk of our universe. Here are a few potential paradigm shifts that we may have to contend with.
Violation of the Copernican principle At the moment we assume that the Milky Way does not occupy any special location within the universe. But if we happen to be living in the middle of a large, underdense void, then it could explain why the type 1a supernovae (our strongest evidence for cosmic acceleration) look dim, even if no form of dark energy exists. However, requiring our galaxy to occupy a privileged position goes against the most basic underlying assumption in cosmology.
Is dark energy something other than vacuum energy? Although vacuum energy is mathematically equivalent to Λ, the value predicted by fundamental theory is orders of magnitude larger than observations can possibly permit and there is no accepted solution to this problem. Many interesting ideas have been proposed, including time-varying dark energy, but even they do not address the “coincidence” problem of why the present epoch is so special.
Modifications to our understanding of gravity It may be that we have to look beyond general relativity to a more complete theory of gravity. Exciting new developments in “brane” theory suggest the influence of extra spatial dimensions, but it is likely that the mystery of dark energy and cosmic acceleration will not be solved until gravity can successfully be incorporated into quantum field theory.
The multiverse Λ can have a dramatic effect on the formation of structure in the universe. If Λ is too large and positive, it would have prevented gravity from forming large galaxies and life as we know it would never have emerged. Steven Weinberg and others used this anthropic reasoning to explain the problems with the cosmological constant and predicted a value for Λ that is remarkably close to what was finally observed. However, this use of probability theory predicted an infinite number of universes in which Λ takes on all possible values. Many scientists mistrust anthropic ideas because they do not make falsifiable predictions and seem to imply some sort of life principle or intention co-existing with the laws of physics. Nevertheless, string theory predicts a vast number of vacua with different possible values of physical parameters, and to some extent this legitimates anthropic reasoning as a new basis for physical theories.
At a glance: The paradigm shift to dark energy
Dark energy is a mysterious substance believed to constitute 75% of the current universe. Proposed in 1998 to explain why the expansion of the universe is accelerating, dark energy has negative pressure and causes repulsive gravity
Data suggest that dark energy is consistent (within errors) with the special case of the cosmological constant (Λ) that Einstein introduced in 1917 (albeit for a different reason) and then abandoned. Λ can be interpreted as the vacuum energy predicted by quantum mechanics, but its value is vastly smaller than anticipated
During the 20th century, Λ was reintroduced a number of times to explain various observations, but many physicists thought it a clumsy and ad hoc addition to general relativity
The rapid acceptance of dark energy a decade ago was largely due to the work of researchers in the 1980s and early 1990s who concluded that, in spite of the prejudice against it, Λ was necessary to explain their data
We still have no fundamental explanations for dark energy and dark matter. The next paradigm shift could be equally astonishing and we must be ready with open minds
More about: The paradigm shift to dark energy
L Calder and O Lahav 2008 Dark energy: back to Newton? Astron. Geophys. 49 1.13–1.18
B Carr and G Ellis 2008 Universe or multiverse? Astron. Geophys. 49 2.29–2.33
E V Linder and S Perlmutter 2007 Dark energy: the decade ahead Physics World December pp24–30
P J E Peebles and B Ratra 2003 The cosmological constant and dark energy arXiv: astro-ph/0207347v2
This is one of those sites where the name says it all. Physics-GamesDotNet is a one-stop shop for clever, innovative and sometimes silly games that feature physically realistic actions and effects. Common formats include bridge-building games, demolition games, brick-stacking games, catapult games and games that require players to move objects from one place to another using levers, inclined planes, rollers and other simple mechanisms. This may not sound like groundbreaking stuff – anyone for a game of Pong? – but closer inspection reveals some surprisingly sophisticated behaviour. Thanks to software that was once the preserve of scientific simulations, the towers in these games totter and tip before they fall over; rolling balls slow down on rough surfaces; and bridges give way under heavy weights. The result is a cross between a game and a basic physics lesson. It is not quite educational, but it is hardly mindless entertainment either.
Can you give me some examples?
Most readers will be familiar with the game Tetris, which requires players to manoeuvre differently shaped blocks into position. The Physics-GamesDotNet variant, 99 Bricks, uses the same set-up, but here the resulting stack is inherently unstable; players must build carefully to ensure that their tower stays upright. Another game, Water Werks, is like a liquid version of pinball: players use the pressure from a (virtual) jet of water to turn wheels and activate springs that guide balls towards an exit. And then there are some games that defy easy categorization. In Home Sheep Home, for example, players must solve physics-based puzzles to guide Shaun the Sheep and his woolly companions back to their barn.
Who created the site?
The site’s administrator is Jeremy Oduber, a student at the University of Amsterdam who has been interested in both science and games since he was a child. He finds physics games particularly appealing because “you can see physics happening all around you every day”. Moreover, he believes that games that incorporate realistic physics tend to be more open-ended than those that do not – meaning that there is usually no “best” way of beating a game or completing a level.
But how much physics is there, really?
It depends on how you look at it. To the casual gamer, the answer is probably “not much”. Although some bridge-building games do provide qualitative feedback on stresses and strains, anyone who wants numerical values for, say, a virtual object’s mass would be better off using a stand-alone physics simulator like Algodoo (see “Web life: Phun“). However, those who dig a little deeper into the world of physics simulation may be surprised at just how much complexity is involved in these relatively simple games. A typical physics simulator, or “engine”, incorporates both gravity and some kind of collision-response mechanism when solving the equations of motion for virtual cannonballs, blocks and so on, while more sophisticated engines also factor in rotations. Not too long ago, only supercomputers could perform such calculations rapidly enough to simulate realistic-looking physical behaviour. So a better answer to the question might be “quite a lot, actually – you just have to look for it”.
Who designs the games?
The games Oduber selects have been developed by people all over the world, from professional designers to teenagers working out of their bedrooms. To appear on the site, games must be bug-free and fun to play – and, of course, they must incorporate physics.
Who is it aimed at, and why should I visit?
Games like the ones on this site are, in Oduber’s view, “great at illustrating some basic concepts of physics in a fun way”. For younger children, we agree with him – particularly if, as is often the case, hands-on alternatives to cartoon wheels and levers are unavailable. But students with exams looming this month should not treat a few rounds of Crush the Castle as a substitute for reviewing their physics notes. Apart from anything else, the games on this site are amazingly addictive. We challenge readers to navigate the gravitational fields in Cosmic Crush or shoot their way through the levels in Ragdoll Cannon without feeling a little rush of excitement. Go on. Try it.
For over 70 years astrophysicists have speculated what might compose the missing dark matter that seems to make up over 80% of all mass in the universe. The typical candidates are fundamental entities known as weakly interacting massive particles, or WIMPs, but new research suggests something more peculiar would better fit the bill.
According to Kathryn Zurek of the University of Michigan and colleagues, “quirky composite dark matter” could explain the universe’s missing mass, but would be free of some of the usual problems associated with conventional WIMP dark matter. “People are becoming more and more open to more complex theories of dark matter,” says Zurek.
WIMPs are so called because they interact with normal “baryonic” matter only through gravity and the weak nuclear force. But after decades of searching, the WIMP hypothesis is starting to come under increasing scrutiny. First, direct-detection experiments – such as the XENON100 experiment at the Gran Sasso laboratory in Italy, or the CDMS experiment in the US – have not yet found any convincing evidence for the existence of WIMPs. This has led to the conclusion that WIMPs must interact exceptionally weakly with normal matter.
Perhaps more importantly, though, there seems to be no reason why dark matter is just four times more abundant than normal matter – in other words, why it has abundance on the same order of magnitude. Since astrophysicists are used to dealing with differences in orders of magnitude of 10, 20 or more, this feature looks like something of a coincidence.
A quirky solution
Quirky dark matter offers a way out of the interaction and coincidence problems. Hypothesized in 2008 by US physicists Junhai Kang and Markus Luty, “quirks” are similar to the quarks that make up in the nucleons inside atoms in that they bind together into composite particles. However, quirks would be much heavier, and rather than be bound by the nuclear strong force, they would be bound by a new type of force – a “dark” strong force. When two oppositely charged quirks bind together they would form, similar to a neutron, a neutral particle. In this latest research, Zurek and her team have developed this theory into a more comprehensive model of dark matter.
It is the innate charge of quirks that enables them to avoid the WIMP coincidence problem. Charge links the quirk abundance to the same processes in the Standard Model of particle physics that determine baryon abundance, so there should naturally be a roughly balanced ratio of quirks (dark matter) to baryons (normal matter). And the composite’s overall neutrality, both in charge and electroweak coupling, would explain why direct detection experiments have so far failed to dig up any evidence.
Neal Weiner, a cosmologist and particle physicist at New York University, says that Zurek has come upon an interesting model. “She and her collaborators have taken these [issues with conventional WIMP dark matter] and really made some headway with them,” he says. “But the best thing is that she has shown how these models can be relevant for experiments. This is not just a theoretical exercise – if any of these ideas are right, we may learn soon.”
Read between the lines
One of the ways experiments could detect quirky dark matter results from the exact nature of each quirk in the composite. If they are just slightly different – say, in mass – the quirks might rarely exchange a photon with an atom’s proton or neutron, and in this way create a nuclear recoil in one of the conventional direct-detection experiments. Moreover, the ability to absorb photons could mean that quirky dark matter has an array of absorption lines, rather like the “Lyman” lines for hydrogen. If the light from a distant source such as a quasar were to shine through a clump of quirky dark matter en route to Earth, telescopes should see these absorption lines in the light spectrum.
As Zurek herself points out, this sort of detection would rely on a bright enough light source and a dense enough clump of quirky dark matter. But with experiments failing so far to find any convincing evidence of conventional WIMP dark matter, physicists may find they have to start exploring more esoteric possibilities. “There could be dark forces, there might be multiple scales in the dark-matter sector, there could be this kind of structure where there’s these excited states…all of a sudden we’re starting to think in terms of much more complex dynamics,” says Zurek.
First there were atomic clocks that beat at microwave frequencies. Then along came optical clocks that provide higher frequency standards. Now, physicists in the US have unveiled plans to build the first “nuclear clock” that runs at still higher frequencies. And because it is based on a solid material, the team claims that such a frequency standard could be far less complicated than gas-based atomic and optical clocks – while delivering the same or better accuracy.
In an atomic or optical clock, electromagnetic radiation is shone on atoms that have an electronic transition involving the absorption of radiation in a very narrow frequency range. A feedback loop locks the frequency of the radiation source to that of the transition, thus creating a very stable frequency standard.
The accuracy of such a clock depends on the intrinsic width of the transition, the frequency of the transition and the experimentalist’s ability to minimize thermal fluctuations and other noise. Today, the best atomic clock has an accuracy of about one part in 1015 while an optical clock has surpassed one part in 1017 – thanks in part to a much higher operating frequency.
Thorium transitions
Now, however, the THOR collaboration including Eric Hudson and colleagues at the University of California, Los Angeles, along with physicists at Yale University and Los Alamos National Laboratory, have started to build the first frequency standard based on a nuclear transition in thorium-229. This transition is extremely narrow and occurs at about 7.6 eV, corresponding to vacuum ultraviolet light.
According to Hudson, the team plans to use a vacuum ultraviolet (VUV) frequency comb as their source of radiation. Already developed by teams at JILA and the University of Arizona, these combs provide light at a series of very precise frequencies. They could also be used to convert a very stable VUV signal into a very stable microwave signal. This is essential for comparing the output of different frequency standards because it is much easier to transfer a microwave signal between labs than a VUV signal.
The nuclear clock also differs from existing atomic and optical timekeepers – which use dilute gases – because the thorium nuclei will be embedded within a solid material.
“Because nuclear transitions are so insensitive to their environment, we believe we can build a nuclear-transition-based frequency reference by simply doping thorium into high quality crystals,” says Hudson. “Instead of the complicated room-sized implementations of current atomic and optical clocks, a nuclear frequency reference would consist of single, possibly room temperature, crystal.”
Performance limits
However, Hudson adds that a limit on the performance of their solid-state design could arise because of tiny shifts in the nuclear transition energy caused by interactions with nearby electrons. Since the electrons are sensitive to changes in the temperature of the crystal, so could the shifts in the nuclear transition energy.
“Unfortunately, even the most sensitive calculations cannot accurately predict the magnitude of this effect; therefore we will simply have to build the clock and measure how much its period depends on temperature”, says Hudson. However, he adds that even in the “worst case scenario”, the device should perform as well as current atomic clocks.
Experimental programme
The team has already carried out experiments to determine the best host crystal for the thorium, with CaF2 and LiCaAlF6 showing the most promise. The physicists then grew several LiCaAlF6 crystals doped with the much more common thorium-232 and studied their VUV properties to ensure that there are no unexpected background effects that could mask the nuclear signal.
Although satisfied that LiCaAlF6 is suitable, the team must now secure a supply of thorium-229, which currently costs a staggering $50m per gram. “We are working with our Los Alamos collaborators to open a new thorium-229 supply line through extraction from old uranium samples”, says Hudson.
Varying constants
Hudson adds that the design could also be used to look for variations in physical constants such as the fine-structure constant or the ratio of the quark masses. “An exciting feature of a nuclear transition is that it is roughly six orders of magnitude more sensitive to any fundamental constant variation than the electronic transitions used in atomic or optical clocks”, he says. “Thus, even with modest measurement accuracy we expect to be able to put severe limits on any variation of the fundamental constants.”
Patrick Gill of the UK’s National Physical Laboratory describes building such a frequency standard as “very challenging”, and points out that the exact frequency of the thorium-229 transition is not yet known. This means that the team could end up spending a great deal of time and effort just searching for the transition before it addresses other experimental obstacles.
View of the Tevatron at Fermilab. (Image courtesy of Fermilab)
By James Dacey
With the sheer scale of its machinery and its extensive international collaborations, accelerator physics is now a highly visible part of mainstream science. A recent episode at Fermilab reminded me, however, that the scientific results could never be as clear-cut as the facilities that produced them. I couldn’t help but feel a little bit frustrated by this, but perhaps that says more about my short concentration span.
Earlier this week, we reported new findings from Fermilab’s D0 experiment, which claimed to have gathered the strongest evidence yet for CP violation beyond the Standard Model. CP violation helps to explain the fundamental difference between the behaviour of a particle and its antiparticle. It explains why matter survived in the universe after the Big Bang, when matter and antimatter were created in equal amounts and should have annihilated completely.
So as you can see the implications of this research were fairly humongous. Hence physicsworld.com and a whole load of other sites covered the story.
However, last Monday – without us realizing – a rival experiment at the Tevatron had already poured cold water on the D0 celebrations by saying that the results were void. Let me explain.
D0 had looked for asymmetry in the production of muons from the decay of B mesons and anti B mesons, and they reported a CP violation that was 3.2 standard deviations larger than what is predicted by the Standard Model.
The particle physics community got very excited because their results had included decent measurements, for the first time, of the decay of Bs mesons, which theorists have long touted as an excellent place to look for CP violation. It seemed that the physics was finally moving beyond the Standard Model.
But before D0 had time to wind down their celebrations, Gavril Girgiu of the CDF experiment was in Turin addressing a particle physics conference about the CDF analysis of the same meson decays. The difference was that CDF had seen nothing out of the ordinary in their results, and their sample size was twice as large – from 5.2 femtobarn of Bs decays, they record CP violation that is within 0.8 standard deviations of the Standard Model.
Now, don’t get me wrong here. I fully realize that experimental physics can only ever move forwards by the proposition of new phenomena followed by its confirmation by other experiments. But, given the expectation that surrounds particle physics – fuelled largely by the high profile of its facilities – I just couldn’t help but feel a bit frustrated by these events. It was just a reminder that despite all the exciting questions and mind-blowing implications of particle physics, the real science so often boils down to more mundane concepts like statistical significance.
Perhaps there is a better way for the particle physics community to communicate that this part of the science can be fun too?
The danger is that people like me, who are intrigued by this weird and fascinating area of science, have come to expect every new result to be as certain as the facilities that have produced them.
src=”http://physicsworld.com/blog/veltman.jpg” width=162 height=227> Martinus ? ? Veltman
It’s Friday afternoon here at Physics World HQ, and my colleagues and I were just looking at the programme for the 2010 Nobel Laureate Meeting in Lindau, Germany — an annual bash where Nobel prize-winners and students from around the world meet up to chew the fat and think big thoughts.
This summer’s meeting features laureates from all disciplines but there are stacks of physicists among them.
The programme looks great — but what caught our eye was that German organizers have very kindly included the middle names of all the laureates who are speaking.
So here’s our quiz for today. Just for fun, it’s your job to guess the middle names of the following laureates. In each case we’ve given a clue.
John Mather — shared the 2006 prize with George Smoot for their work on the anisotrophy of the cosmic background radiation (CMB)
Clue: think English Civil War.
George Smoot — see above
Clue — think The Great Gatsby
Robert Wilson — shared the 1978 prize for discovering the CMB
Clue — think a US president with the same last name.
James Cronin — shared the 1980 prize for symmetry violation in K-mesons
Clue — think a Nobel prize-winning biologist with the same first name.
Martinus Veltman — shared the 1999 prize for electroweak interactions
Clue — he has two middle names. Er, think Latin.
Robert Laughlin — shared the 1998 prize for fractional quantum fluids.
Clue — what he does if he puts money on it.
Add your comments below. I’ll update the blog in a day or two with the answers. In the meantime, no Googling — where’s the fun in that?
Update: Monday 31 May
OK so here are the answers: John Cromwell Mather, George Fitzgerald Smoot, Robert Woodrow Wilson, James Watson Cronin, Martinus Justinus Godefriedus Veltman, Robert Betts Laughlin.
What’s next, a quantum commentator? (Courtesy: UKZN)
By Hamish Johnston
What do quantum cryptography and the World Cup have in common?
The answer is that both will feature at the Moses Mabhida Stadium in Durban, South Africa, next month.
The eThekwini Municipality, which includes Durban, and the Centre for Quantum Technology at the University of KwaZulu-Natal (UKZN) have joined forces to install a quantum-cryptography secured telecoms link between the football stadium and the FIFA World Cup Joint Operation Centre in Durban.
The system includes a dedicated optical fibre connection and the Cerberis quantum encryption system. Developed jointly by Australia’s Senetas and Geneva-based idQuantique, Cerberis is a quantum key distribution (QKD) system that allows two parties to secretly share random strings of bits that are used to encrypt and decrypt messages.
The bits are encoded into a stream of photons that are sent down a dedicated fibre. Anyone trying to intercept the string must make a measurement on the photons. Quantum mechanics dictates that a measurement will cause an irrevocable change to the photons, which would alert the two parties to the presence of an eavesdropper.
According to UKZN physicist Abdul Mizra, the system will be used to ensure the secure transmission of voice and data, including e-mail.
So why do World Cup organizers need such a high level of security?
“With all high-profile events, there is a security concern with regards to dignitaries and teams”, explained Mizra.
I suppose that’s fair enough, given that Barack Obama might put in an appearance.
However, avid physicsworld.com readers will recall that just last week Canadian physicists claimed to have hacked an idQuantique system.
I’m not talking about physical look-a-likes here, but researchers who share your name – and thus might be mistaken for you when someone searches the scientific literature.
In my case, I can’t say that the prospect worries me very much. Although I am only the fifth-most-cited “M L Harris” on the ISI Web of Knowledge – lagging behind a Mary L Harris who studies bowel disease; a Meghan L Harris who works for the Iowa Department of Public Health; and two unspecified M L Harrises who study lung problems in London and environmental toxins in Canada – my doppelgangers and I work in such different fields that it would be hard to confuse us.
However, I can accept that modern-day A Einsteins – of whom there are a surprisingly large number – might feel differently on the issue. The same goes for a physicist I met at a conference once: his name was Slobodan Milosevic.
Name confusion is a particular concern for people whose names have multiple accepted English transliterations, like Xu/Hsu or Müller/Mueller. Different publishers’ conventions can mean that such people become, in effect, their own scientific doppelgangers, with multiple database identities that actually refer to the same person. A separate-but-related problem arises when researchers change surnames after marriage, or add a second initial. Stephen Hawking, for example, publishes as both “Hawking S” and “Hawking S W”. Although he’s probably too famous to care now, lesser-known researchers can suffer if name confusion means that hiring committees and potential collaborators get an incomplete picture of their work.
To address this problem, various organizations have sponsored initiatives that attempt to
assign unambiguous identities to scientific researchers. Many scientific publishers – including IOP Publishing, which publishes physicsworld.com – are working on their internal author databases, trying to eliminate duplicate entries and create clear and accurate records of each scientist’s work.
However, the scope of such databases is usually limited to a single publisher. There are a few broader efforts out there, including the American Institute of Physics’ Uniphy service, but so far, there is no “global, cross-sector, cross-institutional system that research institutions and all types of publishers can share”; like the one this article from Chemical and Engineering News suggests is needed.
But is such a vast database really necessary? Or are scientific doppelgangers a minor problem, one that could be solved with better record-keeping on a smaller scale?
