The BOSS survey's detailed galaxy map is helping astronomers hone in on the nature, and existence, of dark energy.
Ever since the Big Bang, the expanding universe has been a battleground between two cosmic forces: gravity, which tries to hold the universe together, and dark energy, a mysterious repulsive energy that pervades all of space.
For the first 8 billion years, gravity dominated, and cosmic expansion gradually slowed. But as the matter in the cosmos thinned out, gravity’s pull weakened, and about 6 billion years ago dark energy began winning the tug-of-war. The cosmic expansion stopped slowing and started to accelerate. That’s the situation today and, as best as we can tell, forevermore. But until we know for sure what dark energy actually is, the jury remains out.
Now, a project called the Baryon Oscillation Spectroscopic Survey (BOSS) is narrowing down the nature of dark energy with an amazing degree of precision. Astronomers presented the preliminary results from six papers at the National Astronomy Meeting in Manchester, England.
The preliminary results alone cover 130 pages, so here are the Cliff’s Notes:
Mapping the Visible Universe
The BOSS survey contains hundreds of thousands of galaxies. Viewed on scales of a hundred million light-years, galaxies are organized into gigantic, cobwebby structures — crowded walls and filaments separated by enormous, barren voids.
Astronomers can use this map to learn about the universe when it was much younger. Way before the first stars and galaxies had formed, light and matter coexisted in a hot, primordial soup. Huge sound waves rang through this soup, sloshing matter into higher-density and lower-density regions.
As the universe cooled and the earliest galaxies formed, an echo of these acoustic waves remained, imprinted into the galaxies’ stringy arrangements. This imprint acts as a standard ruler, a measurement with which astronomers can measure the expansion of the universe.
Measuring Dark Energy
The strangeness of dark energy might make astronomers squirm in their seats, but every measurement that’s been tried confirms its existence. The BOSS project is no exception — from the observations, astronomers conclude that dark energy makes up between 70.7 and 73.1 percent (at a 68% confidence level) of the universe’s matter-and-energy tally.
The BOSS papers also confirm that whatever dark energy may be, it appears to be unchanging over time: w, which quantifies dark energy’s pressure and density, is a constant of -1.03 (give or take 0.07). Neither of these results is surprising, but the degree of precision is unprecedented: the uncertainty is about five times smaller than previous studies.
“This is quite beautiful work that is delivering on the BOSS promise to provide precision constraints on dark energy and cosmological parameters,” says Niel Brandt (Penn State University), a BOSS scientist who was not involved in the most recent set of papers.
Testing Modified Gravity
Dark energy is called “dark” only because scientists don’t understand what it is. So, what if dark energy doesn’t exist at all — what if the problem lies with scientists’ understanding of gravity itself?
Well, BOSS tests that too.
If gravity itself becomes repulsive on very large scales, then that’s all scientists need to explain the accelerating expansion of the universe. Repulsive gravity would mean that Einstein’s general theory of relativity — a marvel of durability against every test that’s been thrown at it for nearly a century — breaks down on very large scales, an unsettling possibility.
Beth Reid (Lawrence Berkeley National Laboratory) tested for this breakdown by examining the growth of the bubblelike pattern of large-scale cosmic structures in the BOSS survey. General relativity predicts how fast galaxies should fall toward each other to create these structures. Reid measured the individual velocities of hundreds of thousands of galaxies, and found that they were falling exactly as described by gravity as we know it, even on enormous scales.
“We already knew that the predictions of general relativity are extremely accurate for distances within the solar system,” Reid says, “and now we can say that they are accurate for distances of 100 million light-years.”
It appears dark energy is here to stay.