A multi-telescope survey of millions of supernovae could give cosmologists a better understanding of dark energy and could help explain the origins of large-scale structure in the universe, according to cosmologists Pengjie Zhang and Xuelei Chen of the Chinese Academy of Science. The proposed survey would involve measuring the “peculiar motion” of each exploding star, which would reveal new information about how galaxies are distributed throughout the cosmos.

Galaxies are moving away from one another thanks to the ongoing expansion of the universe. However, this expansion is not the same everywhere and each galaxy has an additional “peculiar motion”, which is caused by the gravitational pull of neighbouring galaxies.

Rather than isolate the peculiar motion of individual galaxies, astronomers try to determine the large-scale peculiar velocity (LSPV), which is the average peculiar velocity of galaxies relative to Earth in a certain region of the universe. They rely on knowing the distances between Earth and each galaxy —and that can be very difficult to measure precisely.

'Standard candles'

Something they can measure accurately, however, is the distance to a type-Ia supernovae — exploding white-dwarf stars that are known as “standard candles” because they always give off the same amount of light. All they have to do is measure the supernova brightness, and the dimmer it is, the farther away it is. But astronomers can also estimate the speed with which the star was moving away from Earth when it exploded by measuring the Doppler shift (or “red shift”) in the wavelength of its light.

If the universe was expanding in a uniform manner, all type Ia supernovae of a specific brightness would have the same redshift. However, Chen and Zhang expect that the average brightness of supernovae with the same redshift in a small patch of the sky would be different to the average brightness (at that redshift) over the entire sky. “This difference is proportional to the LSPV,” explains Chen (Phys Rev D 78 023006) .

Chen and Zhang believe that such a survey would be performed best using supernovae with an “intermediate” redshift of about 0.5, which dates their explosions to about five billion years ago. The survey would involve about 106 supernovae, and would require several decades of continuous observation using a number of different telescopes.

Christopher Gordon, a cosmologist at the University Oxford, UK, told physicsworld.com that the proposal “is an interesting idea”. However, he believes that Chen and Zhang may be overly optimistic in saying that such a survey could be completed in several decades – even when new telescopes designed specifically for finding supernovae such as the Large Synoptic Survey Telescope (LSST) come online in the next decade.

Smoothing dark energy

Still, if Chen and Zhang can persuade astronomers to do the survey, the results could shed light on the nature of dark energy — a mysterious substance that counteracts gravity and encourages galaxies to move apart. Dark energy tends to prevent galaxies from clumping and therefore should smooth out variations in the LSPV. However, some theories of dark energy predict less clumping than others.

A survey could also further our understanding of dark matter — a substance that interacts via gravity but not via electromagnetic radiation. Galaxies are made up mostly of dark matter, so Chen and Zhang believe that a LSPV survey could reveal much about the distribution of dark matter in the universe.

The survey would allow physicists to study the effects of gravity over very large length scales and could also help cosmologists understand how primordial quantum fluctuations that existed just after the Big Bang became the large-scale structure of galaxies that we observe today.