Dark Energy: A map hints at what it is – but deepens the controversy over the rate at which the universe is expanding

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Using the Sloan Digital Sky Survey Telescope, researchers have measured more than two million galaxies and quasars – very bright and distant objects that are driven by black holes – in the past two decades. This new map includes around 11 billion years of unexplored cosmic history

By Julian Bautista, Research Fellow, University of Portsmouth

Sloan Digital Sky Survey Telescope. Sloan Digital Sky Survey / wikipedia, CC BY ‑ SA

The dark energy is one of the biggest mysteries in science today. We know very little about it, except that it is invisible, it fills the entire universe and it pushes galaxies and distances them from each other. It causes our universe to expand at an accelerated rate. But what is it? One of the simplest explanations is that it is a “cosmological constant” – a result of the energy of the empty space itself – an idea presented by Albert Einstein.

But this explanation does not satisfy many physicists. They want a more thorough description of its nature. Is it some new type of energy field or exotic liquid? Or is it a sign that Einstein’s gravitational equations are somehow incomplete? In addition, we do not really understand the current rate of expansion of the universe.

Now our project – the Byron Oscillation Extended Spectroscopic Survey (eBOSS) has found some answers. Our work has been published as a series of 23 publications, some of which are still being peer-reviewed, depicting the largest three-dimensional cosmological map ever created.

Now, the only way we can feel the presence of dark energy is through observations of the distant universe. The farther galaxies are, the younger they look to us. This is because in light they emit it took millions or even billions of years to reach our telescopes. Thanks to this kind of time machine, we can measure different distances in space at different cosmic times, and this helps us to calculate the speed of expansion of the universe.

Using the Sloan Digital Sky Survey Telescope, we have measured more than two million galaxies and quasars – very bright and distant objects that are driven by the same black holes – in the last two decades. This new map includes around 11 billion years of cosmic history that has not actually been explored, and we have learned about dark energy as we have never learned.

Our results show that about 69% of the energy in our universe is dark energy. They also show, again, that Einstein’s simplest dark form of energy – the cosmological constant – is most appropriate to our observations.

When combining the information from our map with other cosmological tests, such as cosmic background radiation – the light left over from the Big Bang – they all seem to prefer the cosmological constant over more exotic explanations of dark energy.

Controversial cosmic expansion

The results also provide better insights into some recent controversies about the rate of expansion of the universe today and the geometry of space.

When we combine our observations with studies of the universe in its infancy, cracks are discovered in our description of its evolution. In particular, our measurement of the current rate of expansion of the universe is about 10% lower than the value found using direct methods for measuring distances to nearby galaxies. Each of these two methods claims that its result is correct and very accurate, so that the difference between them cannot be simply statistical randomness.

The expansion of the universe over time. NASA / WMAP Science Team / Graphics: Dana Berry
The expansion of the universe over time. NASA / WMAP Science Team / Graphics: Dana Berry

The accuracy of eBOSS exacerbates this crisis. There is no explanation that is widely accepted in this language. Someone may have made a slight mistake in one of these studies. Or maybe it’s a sign that we need new physics. One of the exciting possibilities is that a form of matter that was not previously known from the early universe may have left traces in our history. It is called “early dark energy”, which is thought to have existed when the universe was young, and could have changed the rate of expansion of the universe.

Recent studies on cosmic background radiation suggest that the geometry of space may be curved rather than simply flat – and this is consistent with the most common theory of the Big Bang. But the conclusion from our study is that the space is indeed flat.

Even after these important developments, cosmologists around the world will still wonder about the apparent simplicity of dark energy, the flatness of space, and the controversial values ​​of the rate of expansion today. There is only one way to move forward in the journey of finding answers – to make larger and more accurate maps of the universe. Some projects are designed to measure at least ten times more galaxies than we measured.

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

For an article in The Conversation

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