map gives clues to what it is, but deepens dispute over cosmic expansion rate

Dark energy is one of the greatest mysteries in science today. We know very little about it, other than that it is invisible, it fills the entire universe and distances galaxies from each other. This is causing our cosmos to expand at an accelerated rate. But what is this? One of the simplest explanations is that it is a “cosmological constant”, a result of the energy of empty space itself, an idea put forward by Albert Einstein.

However, many physicists are not satisfied with this explanation. They want a more fundamental description of their nature. Is it some kind of new energy field or exotic fluid? Or is it a sign that Einstein’s gravity equations are somehow incomplete? Also, we don’t really understand the current rate of expansion of the universe.

Now our project, the extended Baryon Spectroscopic Oscillation Survey (eBOSS), has come up with some answers. Our work has been published as a series of 23 publications, some of which are still being peer-reviewed, describing the largest three-dimensional cosmological map ever created.

Currently, the only way we can sense the presence of dark energy is through observations of the distant universe. The further away the galaxies are, the younger they seem to us. This is because the light they emitted took millions or even billions of years to reach our telescopes. Thanks to this type of time machine, we can measure different distances in space at different cosmic times, which helps us determine how fast the universe is expanding.

Using the Sloan Digital Sky Survey telescope, we measured more than 2 million galaxies and quasars, extremely bright and distant objects that work with black holes, in the past two decades. This new map covers around 11 billion years of cosmic history that was essentially unexplored, teaching us about dark energy like never before.

Our results show that approximately 69% of the energy in our universe is dark energy. They also demonstrate, once again, that Einstein’s simplest form of dark energy, the cosmological constant, is most in line with our observations.

By combining the information on our map with other cosmological probes, such as the cosmic microwave background (the light that remains from the big bang), everyone seems to prefer the cosmological constant over more exotic explanations of dark energy.

Cosmic expansion in dispute

The results also provide a better insight into some recent controversies about the expansion rate of the current universe and about the geometry of space.

Combining our observations with childhood studies of the universe reveals cracks in our description of its evolution. In particular, our measurement of the current rate of expansion of the universe is approximately 10% less than the value found using direct methods of measuring distances to nearby galaxies. Both methods claim that their result is correct and very precise, so their difference cannot simply be a statistical fluke.

EBOSS precision improves this crisis. There is no widely accepted explanation for this discrepancy. It may be that someone has made a subtle mistake in one of these studies. Or it may be a sign that we need new physics. An exciting possibility is that a previously unknown form of matter from the early universe could have left a trace in our history. This is known as “early dark energy,” believed to be present when the universe was young, which could have altered the rate of cosmic expansion.

Recent studies of the cosmic microwave background suggested that the geometry of space may be curved rather than simply flat, which is consistent with the most widely accepted big bang theory. But our study concluded that space is flat.

Even after these major breakthroughs, cosmologists around the world will be puzzled by the apparent simplicity of dark energy, the flatness of space, and the controversial values ​​of today’s rate of expansion. There is only one way to go in the search for answers: to make bigger and more detailed maps of the universe. Several projects aim to measure at least ten times more galaxies than we do.

If eBOSS maps were the first to explore a gap that was previously missing 11 billion years from our history, the new generation of telescopes will make a high-resolution version of the same time period. It’s exciting to think about the fact that future surveys will be able to solve the remaining mysteries about the expansion of the universe in the next decade or so. But it would be just as exciting if they revealed more surprises.

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