Did a discovery of dark energy just prove that Einstein was wrong? Not enough.


The largest study of galaxies ever done suggests that our cosmos is not as dense as it is supposed to be. This lack of lumps could mean there’s a gap with Einstein’s general relativity theory, which scientists use to understand how the structures of our universe have evolved over 13 billion years.

“If this disparity is true, then maybe Einstein was wrong,” said Niall Jeffrey, one of the co-heads of the Dark Energy Survey (DES) and cosmologist at the École Normale Supérieure, in Paris, told BBC News

The DES team compiled a catalog of hundreds of millions of galaxies and used tiny distortions in the shape of these galaxies to measure the universe’s vital statistics. Almost all of these measurements confirmed the big Bang model of cosmology, in which all of the matter in the universe has expanded from an incredibly hot and incredibly small point.

Related: From the Big Bang to the present day: Snapshots of our universe through time

But one of those measurements – the aggregation of matter – was a little flawed. If the universe is smoother than expected, it would mean that our understanding of the evolution of structures in the universe, which is based on Einstein’s general theory of relativity, would be flawed.

While some headlines already claim that Einstein was wrong and that physicists need to revise their models, the reality is much more nuanced. This is because the gap is not yet a statistical slam dunk.

The biggest poll ever

More than 400 scientists from 25 institutions in seven countries are working on DES, one of the largest astronomical collaborations in history. The team used the 4-meter (13.1-foot) Victor M Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile to observe one-eighth of the entire night sky during 758 nights of observation.

The observation project started in 2013 and ended in 2019. But the observation was the easiest part: the DES collaboration took two years to publish its latest results, which only take into account data from first three years of observations.

And it is bluffing.

The release, described in an avalanche of 29 scientific papers, contains detailed observations of 226 million galaxies, making it the largest and most detailed survey of galaxies in history.

This huge catalog still only represents less than a tenth of a percent of all the galaxies in the observable universe, but it’s a start.

The new study relied on this 570-megapixel dark-energy camera on the 4-meter Victor M. Blanco telescope, shown here at the Inter-American Observatory at Cerro Tololo in Chile. (Image credit: Reidar Hahn, Fermi Lab)

Take the measure of the cosmos

DES used its treasure trove of galaxies to study two main features of our cosmos. One is called the cosmic web. As it turns out, galaxies aren’t randomly scattered across the universe, but rather organized around the largest pattern found in nature. At the largest scales, astronomers find giant clusters of galaxies called clusters, long filaments of galaxies, wide walls, and vast empty cosmic voids.

The cosmic web is a dynamic object, and it has evolved to its present state over billions of years. Astrophysicists believe that a long time ago, matter in the universe was much more evenly distributed. By studying the evolution of the cosmic web, DES scientists can understand what the universe is made of and how it behaves. This is because the contents of the universe dictate its evolution, just as changing the ingredients in your favorite cake recipe changes the way it comes out of the oven.

DES also uses something called a weak gravitational lens. We know from Einstein’s general theory of relativity that an object gravity can bend the path of light. The most famous examples of this come from galaxy clusters; their incredible mass can distort the light of background galaxies so much that these galaxies appear very stretched and elongated to observers.

Related: 8 ways to see Einstein’s theory of relativity in real life

DES uses a much more subtle version of this lens effect. He searches for tiny changes in the shape of galaxies due to the light from these galaxies traveling billions of light years of space. By comparing these galactic shapes to what we know galaxies look like from near universe studies, DES astronomers can map the distribution of matter in the cosmos.

Something is wrong

The DES collaboration compared their results to those of other major studies, such as the Planck study of the cosmic diffuse background, the Big Bang echo revealed in a faint radiant glow that permeates the universe. Their results corresponded almost perfectly to existing observations and to the dominant cosmological theory: we live in an expanding universe about 13.7 billion years old, whose mass-energy is made up of about a third of matter (of which most of it is black matter), with the rest in dark energy.

But one metric stood out: a parameter called S8, which characterizes the amount of lumps in the universe. The higher the value of S8, the more tightly the material agglutinates. The new DES results favor a value for S8 of 0.776, while the older Planck results showed a slightly higher value, 0.832.

Planck’s results come from measurements in the early universe, while DES results come from later in the universe. These two numbers should match, and if they are truly different, then our understanding of the growth and evolution of giant structures over cosmic time – which is based on our understanding of gravity through general relativity theory. Einstein – could be wrong. Because no one expected to find this discrepancy, astrophysicists haven’t explored exactly which parts of relativity may be imperfect.

Make headlines hailing the DES findings as a major flaw in the foundations of our modern cosmological theories. “I spent my life working on this theory [of structure formation] and my heart tells me I don’t want to see it crumble, ”Carlos Frenk, a cosmologist at Durham University in England, who was not associated with DES, told BBC News. “But my brain is telling me that the measurements were correct, and we have to consider the possibility of a new physics.”

But what these headlines (and articles) neglect to mention is uncertainty. Every measurement has an uncertainty – scientists cannot be precise given the amount of data available. When statistical uncertainties are included, DES and Planck results generally overlap. Not much – so the difference is worth exploring – but not enough to sound the alarm. In the language of statistics, the two measurements are only lagged by 2.3 standard deviations, which means that if there really was no real difference between the values ​​of S8 and the observations had to be repeated 100 times, they would give the same (or more) difference 98 times. This is well below the 5 standard deviations usually required to announce a new discovery.

Let’s see what three more years of data brings.

Originally posted on Live Science.


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