Einstein's view of the Universe has gotten some new respect; the most accurate measurement ever made of the distance to the time when the expansion of Universe began to accelerate. It means that the phenomenon can be explained using just Einstein’s general theory of relativity and the cosmological constant - the simplest theoretical explanation for the acceleration of the Universe. The results will be used to understand what is causing the acceleration and why, and could shed new light on 'dark energy' – the adopted name of the little-known fundamental agent driving the acceleration.
Cosmologists from the University of Portsmouth and the Max Planck Institute for Extraterrestrial Physics examined the period between five and six billion years ago when the Universe was almost half its present age and made measurements of extraordinary accuracy - within 1.7 percent. The observations tell us more than ever before about the accelerating expansion of the Universe. The findings support Einstein’s general theory of relativity which predicts how fast galaxies, separated by large distances, should be moving toward one another and at what rate the structure of the Universe should be growing. The conclusions are consistent with the ‘concordance model’ of a Universe that bloomed from the big bang 13.7 billion years ago.
The experiment was designed to follow up on a remarkable observation made in 1998, when scientists studied the brightness of mighty stellar explosions to deduce that the Universe’s expansion is, against all odds and against our understanding of fundamental physics, becoming increasingly fast. The research was awarded with the Nobel Prize for Physics in 2011 and the observation has been puzzling physicists and cosmologists ever since.
The new discoveries are based on work by a collaboration of astronomers from across the globe representing the Baryon Acoustic Oscillation Spectroscopic Survey (BOSS), part of the third Sloan Digital Sky Survey (SDSS-III), which is mapping the three-dimensional positions of over one million galaxies.
CLICK IMAGE FOR LARGER SIZE. The record of baryon acoustic oscillations (white circles) in galaxy maps helps astronomers retrace the history of the expanding universe. Credit: Eric Huff, the SDSS-III team, and the South Pole Telescope team. Graphic by Zosia Rostomian
Professor Will Percival, head of this team at the University of Portsmouth’s Institute of Cosmology and Gravitation, said, “Dark energy is the dominant component of our present-day Universe and yet its origin and nature remain a complete mystery. Its effect means that empty space in our Universe seems to behave in the opposite way to a sponge – if you could squeeze it, its density would go down instead of up. “We know that seven billion years ago dark energy had a much smaller impact on the Universe and one of our aims is to discover what changed – and why. Science often works on first discovering the effect of something, and then investigating the cause.”
Team member Dr Rita Tojeiro said, “The results are the best measurement of an intergalactic distance ever made which means cosmologists are closer than ever to understanding why the Universe's expansion is accelerating.
“One of the great things about Einstein’s general theory of relativity is that it is testable. Our results support the theory and are fully consistent with the notion that constant vacuum energy –empty space creating a repulsive force – is driving the acceleration of the Universe. These are profound statements that describe the physics of our Universe at the most fundamental level.
Critically, the results find no evidence that dark energy is simply an illusion stemming from our poor understanding of the laws of gravity – Einstein’s theory has passed its most stringent test yet at extra-galactic scales. Cosmologists examined over a quarter of a million galaxies within a volume 4 billion light years across. The research was carried out using Baryon Acoustic Oscillation (BAO), a pattern in the way galaxies are distributed across the universe which was created very shortly after the big bang. This pattern survived as the Universe grew older and crucially, the size has increased in line with its expansion.
Percival said, “Imagine dropping a pebble in a pond that is itself expanding. If the expansion of the pond is fast enough, the distance between ripples in the pond simply grows with the expansion of the pond. By looking at how the distances between those ripples changes with time you would be able to judge the rate at which the pond is expanding. Well, the Universe is that pond and the galaxies are the water molecules.”
The University of Portsmouth team played a pivotal role in how the results were discovered. Firstly they worked to remove ‘interference’ from our own galaxy to permit a clearer picture of the galaxies further away. Dr. Ashley Ross, who undertook this part of the work, said: “Observing the Universe from the Earth means seeing it through our own galaxy, which is like looking through a dirty lens that we need to clear in order to have an unobstructed view of what we really want to look at.”
The team then created computer models of alternative universes based on data about our own on which many more experiments could be carried out. The ICG’s Dr Marc Manera said: “Cosmology is not like other sciences – we only have one Universe to study and we can only perform most experiments one single time. We used a super computer to create 'fake' universes that share the same broad characteristics as our own, and this allows us to carry out multiple observations and assess the statistical accuracy of the measurements of the Universe in which we live.”
Finally they measured the acoustic waves and their scale, which formed the basis of the team's analysis.
Percival said: “Dark Energy was only discovered 14 years ago, and there’s this feeling that we are still riding the wave of discovery. It’s a very exciting time to be a cosmologist.”
The Sloan Digital Sky Survey (SDSS) III: is being conducted on a 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico
Marc Manera et al: 'The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: a large sample of mock galaxy catalogues' - presented at this week’s National Astronomy Meeting in Manchester
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