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Astroparticle physics

Microwaves map cosmic origins

05 Jun 2001

Recent data from the cosmic microwave background add further weight to the inflationary big-bang model. But more precise measurements from NASA’s MAP satellite, which is due to be launched this month, could challenge this theory.

DASI
Observing the cool glow from a hot universe ­ the Degree Angular Scale Interferometer at the South Pole (Picture credit: John Yamasaki/University of Chicago).

Cosmologists aren’t shy of making bold claims about their subject. Paul Steinhardt of Princeton University, for example, believes that we are living through a “revolution” in our understanding of the universe as profound as that brought about by Copernicus. His convictions have been bolstered by results announced at the end of April from three independent experiments measuring the so-called cosmic microwave background (CMB). The results show that sound waves propagated through the early universe in a series of harmonics, as predicted by the established theory of the universe’s origins – inflationary big-bang theory.

“The latest CMB measurements are a stunning vindication of our understanding of the universe,” says cosmologist Michael Turner of the University of Chicago. “We are now making measurements of a high enough precision that we can test predictions of bold ideas. These measurements show that inflation is passing with flying colours.”

Inflation was first proposed by Alan Guth of the Massachusetts Institute of Technology in 1981. According to this theory, the very early universe expanded exponentially for about 10-32 seconds, before settling down to a slower rate of expansion. In the process, quantum fluctuations were stretched out into the density variations that eventually led to all the structure in the universe, from galaxies to humans. The three experiments – called Boomerang, DASI and Maxima (an international and two American collaborations, respectively) – detected these density variations in the CMB, the microwave remnant of the radiation produced about 300 000 years after the big bang (see box).

Turner admits that it is “too early to crown the inflationary model”, but is confident that it will see off the competition. The new data rule out several alternative models of the universe’s evolution known as topological defect models, since these theories predict that structure was seeded more than 300 000 years after the big bang.

However, not everyone is so sure about inflation. John Peacock of Edinburgh University, for example, points out that the data do not rule out other theories that propose a quantum origin to structure. “The peaks [in the CMB] were predicted ten years ahead of inflation. To say that they are strong evidence for inflation itself is wishful thinking,” says Peacock. “The CMB is verification that we understand how structure in the universe has developed since the universe was 103 times smaller than at present – but inflation is presumed to have happened when the universe was 1028 times smaller than today, so there is still a long way to go.”

Mike Disney, a cosmologist at Cardiff University, is also sceptical of inflation theory, which he likens to our understanding of the solar system. “The celestial motion of the planets can be traced with wonderful precision but this doesn’t mean we know how the solar system formed,” he says. “I am very impressed with the Boomerang data but this doesn’t mean we know everything about the evolution of the universe.”

A snapshot of the primordial universe

The early universe consisted of a hot, dense plasma containing photons, electrons, protons and a small amount of helium and other light elements. The photons repeatedly scattered off the electrons and were therefore restricted to the plasma. But when the universe was about 300,000 years old it had expanded and cooled to below 3000 K – low enough to allow atomic hydrogen to form. In the absence of free electrons, the photons were able to travel freely through the expanding universe. The background radiation that we detect today is therefore a record of the universe as it was just 300,000 years after the big bang, but it has been stretched, or “red-shifted”, to its current microwave wavelength due to the universe’s expansion.

The density variations caused by inflation manifest themselves as hot and cold patches in the background radiation. This is because the differences in density set up compressions and expansions in the primordial plasma, which propagated as sound waves, causing the gas to heat up when compressed and cool when expanded. The Cosmic Background Explorer satellite first detected these patches in 1992 as temperature variations of just one part in 105 in the microwave radiation, which it showed had a perfect black-body spectrum with a temperature of 2.73 K.

An analysis of the data from the Boomerang and Maxima experiments that was published last year confirmed the presence of the largest patches, which extend about a degree across the sky. But there was no firm evidence for higher “harmonics” – the patches that are predicted to occur at smaller angular scales. These have been seen in the latest data (see image above). They are also represented as peaks in a graph of the amplitude of temperature fluctuations against angular scale, i.e. how many ripples there are of a given size. The first, and largest, peak lies at about a degree, the second at about 0.4 degrees and the third at about 0.25 degrees.

Putting inflation to the test

Altogether over 30 experiments to measure the CMB have been completed, are in operation or are planned. Until now these have been small-scale affairs – some of which have been ground-based, such as DASI, located at the South Pole, and most of the rest – such as Boomerang and Maxima – have been suspended from balloons. But that will change this month with the launch of NASA’s $145m satellite called the Microwave Anisotropy Probe (MAP). This will be some 100 times more accurate than previous experiments and will record images of the CMB across the whole sky. Boomerang, in comparison, covered only about 3% of the sky.

“The idea is to get as much information out of the CMB as possible,” says Turner. “Unlike the balloon experiments, MAP will not suffer from atmospheric interference and will have no calibration problems.” Since it will study the whole sky, MAP will be able to calibrate its voltage measurements using a pair of well known hot and cold spots in the heavens.

Chuck Bennett, the principal investigator on the MAP mission, believes the NASA satellite will significantly improve our knowledge of the universe. “MAP is in a different category to current experiments. It will make a full sky map with unprecedented precision and accuracy to determine the history, content, shape and fate of our universe.”

But the excitement does not stop there. In 2007 the European Space Agency will launch the Planck satellite. It will measure temperature fluctuations some three times smaller than the smallest to be studied by MAP, and will be the first serious attempt to measure the polarization of the CMB.

Measuring the CMB’s polarization will be a major test of inflationary theory. The gravity waves that would have been created by the huge expansion of matter in the inflationary universe lead to a distinctive pattern of polarization. But rival theories would not necessarily involve the formation of gravity waves. The “ekpyrotic” theory, for example, which has recently been put forward by Steinhardt and colleagues, proposes that our universe was kicked into life by an offshoot of a hidden parallel universe subject to random quantum fluctuations. This does not involve a rapid expansion of the early universe and therefore disturbances in the gravitational field have properties that differ from those predicted by inflation.

Planck will also provide another major test of inflation by measuring the so-called scale invariance of the fluctuations in the CMB to better than 1%. (MAP could measure gravity waves or scale invariances in principle, but existing data suggest it may not be accurate enough.) Scale invariance is calculated by measuring the relative amplitudes of the fluctuations in the CMB. If the universe is scale invariant then the size of density perturbations, which are related to the temperature fluctuations, is independent of the angular scale. Inflation predicts that the perturbations will have a small, but measurable, deviation from exact scale invariance.

Precision cosmology

Measuring something to within 1% may not sound like anything to be proud of. In particle physics, for example, researchers need to reduce errors to three parts in 10 million in order to claim the discovery of a new particle. But 1% is a big deal in cosmology, a subject more used to measurements that are in error by several hundred per cent. In fact, the improvement in the measurement of scale invariance that Planck will obtain should be enough to test the validity of inflation.

What gets cosmologists excited about the CMB, however, is not just the accuracy of the measurements in isolation but that the data fit into an overall framework supported by a range of cosmological observations. “We are now in a wonderful position in cosmology because results from different kinds of measurement are fitting together in a coherent whole, such as those from the CMB and supernovae,” says Steinhardt.

By measuring the relative amplitudes of the temperature fluctuations at the largest angles, cosmologists have calculated that ordinary, or “baryonic”, matter accounts for about 5% of the universe’s total mass and energy. This figure agrees with an estimate of the amount of ordinary matter in the universe based on calculating the quantity of deuterium produced in the big bang.

“The agreement between measures of the amount of ordinary matter is simply stunning,” says Turner, “even though the underlying physics is completely different. The big-bang framework and Einstein’s general relativity have passed a major new test.”

The new CMB data also support the idea that the geometry of the universe is flat, which means light travels in straight lines. The universe consequently has a “critical” energy density since any deviation from this density would mean light following a curved trajectory. This is important because it is consistent with observations of the more recent universe.

In particular, observations of galaxies, such as those contained within the “two-degree field” survey being carried out by British and Australian astrophysicists, show that the total amount of matter – baryonic plus exotic dark matter (matter not made up of electrons, protons and neutrons) – is only about one-third of the critical energy density. It is thought that the remaining two-thirds of the energy density is made up of a mysterious substance called “dark energy”, which would act like negative gravity. If the total energy density consisted entirely of matter, the consequent gravitational potential would cancel the kinetic energy of the universe, and the universe’s expansion would eventually grind to a halt. But the dark energy means instead that the universe’s expansion ought to accelerate.

Such an acceleration has indeed been observed in distant supernovae, which ties the loose strands together and makes cosmologists happy. “The dark-energy revolution is as important as the Copernican revolution,” says Steinhardt. “Copernicus challenged our perception of our place in space. Dark energy challenges our place in time.”

This view is not shared by everyone. Joseph Silk, an astrophysicist at Oxford University, stresses that dark energy remains a working hypothesis. “I am not going to work on this,” he says. “I hope dark energy will go away. It has come and gone over the years. It all remains to be confirmed.” He does admit, however, that “the evidence is piling up, and looks more and more convincing”.

Cosmologists are sceptical about dark energy, not least because no-one knows what it actually is. One possibility is the “cosmological constant” that Einstein introduced as a fudge factor in general relativity to explain what was presumed to be a static universe (he described this as his “greatest blunder” following Hubble’s observations of the expanding universe). Another possibility is something known as “quintessence”. Both alternatives would consist of vacuum energy, but the latter would evolve over time. A comparison of the vacuum density today with that imprinted in the CMB should therefore tell researchers the correct model, a calculation that may be possible with measurements from MAP and Planck.

Disney points out that new observations could not only bolster current theories, but may completely overhaul current thinking. “Originally scientists thought we lived in a static universe. But experimental evidence proved otherwise,” he says. “The universe is a more complicated place than theorists like to think. In 20 years time’ we will still be wondering about the big questions.”

Turner agrees that people will still be searching for cosmic answers in 20 years’ time, but believes that inflation, dark energy and cold dark matter will hold firm. “I’m more than cautiously optimistic that inflation theory will still be around come the end of the Planck mission,” he says. “It is true that the only surprise in this golden age of cosmology would be no more surprises. But I don’t think there will be any surprises to topple the whole framework.”

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