Scientists have used the Dark Energy Spectroscopic Instrument (DESI) to create the most detailed 3D map of the universe so far. The catalog includes over 7.5 million galaxies, more than all other redshift surveys combined. These kinds of maps are essential to unraveling the nature of the universe and its evolution. Particularly, scientists are interested in understanding dark energy, the mysterious force thought to be responsible for the accelerated expansion of the universe.
DESI consists of over 5,000 optical fibers mounted on the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory in Arizona. These many fibers allow the instrument to capture detailed color spectrum images of millions of galaxies across more than a third of the sky. By breaking down the spectrum of colors, researchers can determine just how much the light has been redshifted — meaning how much it has been stretched toward the red end of the spectrum by the expansion of the universe.
American astronomer Edwin Hubble, after whom the famous Hubble Space Telescope is named, first linked the redshift phenomenon to the expansion of the universe in the 1920s, writing: “This redshift appeared to be larger for faint, presumably further, galaxies. Hence, the farther a galaxy, the faster it is receding from Earth.”
These redshifts allowed DESI to translate a flat observation of the sky into depth, which researchers use to create a 3D map of the universe. DESI has only been online for seven months — just 10% of the way through its five-year mission — but it has already surpassed in size and scope all other similar surveys before it.
By the time the DESI survey is completed, more than 35 million galaxies will have been mapped. The catalog will provide astronomers with copious amounts of data points, an intricate web of complexity that can reveal both the past and future movements of the universe.
“There is a lot of beauty to it,” said Berkeley Lab scientist Julien Guy during a Berkeley Lab-hosted webinar called CosmoPalooza. “In the distribution of the galaxies in the 3D map, there are huge clusters, filaments, and voids. They’re the biggest structures in the universe. But within them, you find an imprint of the very early universe, and the history of its expansion since then.”
“Our science goal is to measure the imprint of waves in the primordial plasma,” he added. “It’s astounding that we can actually detect the effect of these waves billions of years later, and so soon in our survey.”
Nearly 70% of the content of the universe is thought to be made of dark energy, a mysterious form of energy believed to drive the expansion of the universe at an accelerated rate. As the universe expands, more dark energy is created, which causes the universe to expand even faster — a self-perpetuating cycle that drives the expansion of the universe seemingly to infinity.
But can the universe truly expand infinitely? Or will it collapse onto itself in a reverse Big Bang (Big Crunch)? To answer such existential questions scientists need to know how dark energy has behaved in the past, which is where DESI comes in.
Until dark energy is demystified, DESI data can also advance science in other ways. For instance, cosmologists can use DESI’s maps to verify whether Einstein’s theory of general relativity holds over immense spans of space and time. DESI data is also allowing scientists to understand the behavior of intermediate-mass black holes that are otherwise impossible to spot using conventional telescopes. Quasars, some of the brightest and most distant objects in the universe, are also important targets for DESI.
“It’s pretty amazing,” said Ragadeepika Pucha, a graduate student in astronomy at the University of Arizona working on DESI. “DESI will tell us more about the physics of galaxy formation and evolution.”
During the 20th century, the idea that the Universe existed in a steady state was seriously challenged and eventually dismissed by the discovery that the Universe is not only expanding but is also doing so at an accelerating rate. The reason for this accelerating expansion is thus far unknown, but scientists have given this force a name–albeit one that is a placeholder–dark energy.
Explaining this accelerating rate of expansion has become one of the most challenging problems in cosmology. The fact that the value for this acceleration varies wildly between theory and practical observations has created a raft of problems in itself. This means that the net result arising from any attempt to explain dark energy creates more questions than it answers.
Dark Energy: The Basics
Dark energy is whatever is causing the expansion of the Universe to accelerate. One of the most striking things about this mysterious energy is just how much ‘stuff’ in the Universe it accounts for. If you consider the contents of the cosmos to be matter and energy –more formally known as the Universe’s mass-energy density–then dark energy accounts for between 68% to 72% of all cosmic ‘stuff.’
Dark matter is the second-largest contributor, with just a tiny proportion of the Universe’s ‘stuff’ made up of the baryonic matter consisting of atoms that we see around us on a day-to-day basis.
This is even more staggering when you consider that all the stars, planets, dust, gas and cosmic bodies that make up the visible Universe are contained in this tiny fraction of cosmic stuff that doesn’t even amount to 5% of the Universe’s contents.
Dark energy could very roughly be described as a force acting in opposition to gravity. Whereas the more familiar everyday force of gravity holds objects like planets and stars together in orbits, dark energy acts as a repulsive force, driving galaxies themselves apart. But, whereas gravity acts upon objects themselves, dark energy acts on the very fabric of spacetime between objects. Because dark matter is the largest contributor to gravity, this means this mysterious substance is locked in what has been termed a ‘cosmic tug of war.’ And it’s clear during our current epoch, dark energy is winning!
A popular analogy for this is the description of the Universe as the surface of a balloon. Galaxies are represented by two marker ink dots on this surface. As the balloon is inflated the points move apart with the very space between them expanding.
Repeat the experiment with three unevenly placed dots and it is clear that the dots that are initially further apart recede away from each other more rapidly. This extremely rough analogy carries over to galaxies. The further apart the galaxies are, the more quickly they recede.
An Expanding Surprise
The discovery that the Universe is expanding came as a considerable shock to the scientific community when it was confirmed by Edwin Hubble in 1929. Hubble had built upon theory provided by George Lemaitre and Alexander Friedman who had used the equations of general relativity to predict that the Universe was non-static, something that very much contradicted the scientific consensus of the time.
Albert Einstein who devised general relativity had also found that his geometric theory of gravity predicted a non-static Universe, something he wasn’t exactly comfortable with. Despite having already killed many of the sacred cows of physics, Einstein was unwilling to do away with the concept of a static Universe. To recover a static Universe that was neither expanding nor contracting, the world’s most famous scientist introduced to his equations a ‘fudge factor’ called the cosmological constant–commonly represented with the Greek letter lambda.
The cosmological constant was in danger from the start. Once Hubble managed to persuade Einstein that the Universe was indeed expanding, the physicist abandoned the cosmological constant, allegedly describing it as his ‘greatest blunder’ in his later years. The cosmological constant wouldn’t stay in the cosmological dustbin for very long, however.
If physicists had been surprised by the discovery at the beginning of the 20th Century that the Universe is expanding, they would be blown away when at the end of that same century when the observations of distant supernovae made by two separate teams of astronomers revealed that not only is the Universe expanding, it is doing so at an accelerating rate.
To understand why this is shocking and how it leads to the conclusion that some repulsive force is driving this expansion, it is necessary to journey to the very beginning of time… Let’s take some balloons too…
Escaping the Big Crunch
When thinking about the initial expansion of the Universe it makes sense to conclude that the introduction of an attractive force within that Universe would slow and eventually halt this expansion. That is exactly what gravity should do, and it seems did do during the early stages of expansion that had nothing to do with dark energy (we think).
Some cosmologists are willing to go a step further. If there is no outward pressure but an inward attractive force, shouldn’t the Universe actually start to contract?
This leads to the theory that the Universe will end in what physicists term the ‘Big Crunch’–an idea that dark energy could make obsolete. Think about how counter-intuitive this was to scientists when the idea was first suggested and evidenced. Let’s return to the balloon analogy; imagine you stop blowing into the balloon, and instead start sucking the air out of it.
How shocked would you be to find that the balloon isn’t contracting, it’s continuing to expand? And not just that, it’s actually expanding faster than it was when you were blowing into it!
With that in mind, consider the initial moments of the Universe. Beginning in an indescribably dense and hot state, squeezed into a quantum speck, the Universe undergoes a period of rapid expansion. This period of expansion wasn’t driven by dark energy. As it expands, the Universe cools allowing electrons to form atoms with protons and neutrons, which in turn frees photons to travel the cosmos.
Soon there is enough matter in the Universe to allow the attractive force of gravity to slow its expansion. And this does seem to be what happened in the early cosmos. The rapid inflation of the infant Universe is believed to have halted at around 10 -32 seconds after the Big Bang, with the Universe still expanding, albeit at a much slower rate.
This period of expansion continued to slow as a result of the growing dominance of matter during what cosmologists call the ‘matter-dominated epoch.’ But, at around 9.8 billion years into the Universe’s 13.8 billion year history, something strange begins to happen. The Universe begins to expand again, this time at an accelerating rate.
This is the dawn of the dark energy dominated epoch.
The Cosmological Constant is Back and Still Causing Trouble
That explains why the accelerating expansion of the Universe is so troubling and the need to introduce the placeholder concept of dark energy to explain it. Yet this accelerating expansion would still need a mathematical representation in equations used to describe the Universe. To do this cosmologists would return to the cosmological constant and its symbol, lambda.
This new iteration of the cosmological constant would be used in a different way to Einstein’s version. Whereas the earlier cosmological constant was used to balance gravity and hold the Universe steady and static, this new version would be used to overwhelm gravity and account for the acceleration of its expansion. But, this revised use of the cosmological constant does not mean it is any less troublesome than Einstein had found its predecessor.
In fact, the difference between the cosmological constant’s measured value, found by measuring the redshift of distant Type Ia supernovae, diverges from the value predicted by quantum field theory and particle physics by a value as large as 10121 (that’s 1 followed by 121 zeroes). Thus, it should come as no surprise that this value has been described as the worst prediction in the history of physics.
And as it represents the action of dark energy, that makes dark energy itself cosmology’s biggest conundrum.
OK… But What is Dark Energy?
So by now, you might well be thinking all of this is all fine and good, but this article specifically asks ‘what is dark energy?’ Isn’t it time to get to answering this question? It should come as no surprise that the answer is no one knows. But, that doesn’t mean that cosmologists don’t have some very good ideas.
One of the explanations for dark energy says that it could be vacuum energy, an underlying background energy that permeates that Universe and is represented by the cosmological constant. The most commonly cited evidence for vacuum energy–the energy of ’empty’ space which manifests as the Casimir effect.
Without delving too deeply into this, as relativity states that energy and mass are equivalent and mass has gravitational effects, then it stands to reason if empty space has vacuum energy, this too should contribute to the effect of gravity across the cosmos. That contribution has been factored in as a negative repulsive influence acting against the attractive influence of gravity.
The big problem with this is that quantum field theory suggests that this negative pressure contribution from vacuum energy should arise from all particles and thus, should give lambda a value that is tremendously larger than that obtain when our astronomers measure the redshift of Type Ia supernovae in distant galaxies.
This problem could be solved by dark energy’s effects being the result of something other than vacuum energy, of course. The Universe’s accelerating expansion could be due to some, as of yet undiscovered fundamental force of nature. Alternatively, it could indicate that our best current theory of gravity–general relativity– is incorrect.
A new generation of cosmologists is currently actively tackling the dark energy puzzle with new and revolutionary ideas. These include the idea that dark energy could have started work in the early Universe, an idea proposed by Early Dark Energy (EDE) models of the Universe. Another alternative is that dark energy does not influence the curvature of the Universe, or perhaps does so weakly–a theory referred to as the ‘well-behaved cosmological constant.’
As unsatisfying an answer as it is, the only honest way of addressing the question ‘what is dark energy?’ right now is by saying; we just don’t know. But, science wouldn’t be anywhere near as fascinating without mysteries to solve, and revolutionary ideas to be uncovered.
Gravitational waves, which were first detected only two years ago, could help astronomers come up with a much more precise value for the universe’s rate of expansion. According to researchers at the University of Chicago, an accurate estimate using this method could be developed within five to ten years.
An artist’s impression of gravitational waves generated by binary neutron stars. Credit: R. Hurt/Caltech-JPL/EPA.
In 1929, Edwin Hubble shook the world of astronomy after his observations proved that the universe was not static but that rather it is in a constant state of expansion. Not only this but the rate of expansion is accelerating — the farther away an object is, the faster it will recede from our vantage point. This insight underpins much of modern cosmology and without the notion of an expanding universe, there could be no such thing as the Big Bang theory, for instance.
Yet pinning down the exact rate of expansion, the so-called Hubble constant, has proven challenging work even with today’s modern instruments and techniques, almost a century after Hubble made his groundbreaking discovery.
Astronomers employ two very different methods to calculate the Hubble constant: one involves recording the distance to a faraway object (such as a bright quasar) and how fast it is moving away from us. This second quantity is relatively easy to determine since light increases its wavelength due to universal expansion, shifting into the red part of the spectrum as it recedes — this is the so-called the Doppler effect.
However, measuring distance is much more prone to error because there are many steps where assumptions and approximations are made along the way. Simply put, if you keep rounding up at every step, the minor errors add up and you can eventually end up with a significant error for the final value.
The second approach to computing the Hubble constant is studying the cosmic microwave background radiation — the leftover radiation from the Big Bang, or the time when the universe began, which is still detectable. This method too relies on certain assumptions about how the universe works.
The problem is, when you compare the two, you don’t end up with a similar result.
What you get when you compare the results of the two methods is a 10% difference in value. That’s far too much of a discrepancy, which is why many astrophysicists are on the lookout for a novel method that might come up with a more precise value for the Hubble constant.
Daniel Holz, a University of Chicago professor in physics, and colleagues may just have found the golden ticket. By combining observations of gravitational waves, such as those produced by the collision of two stars, and the light emitted in the aftermath of the collision, the scientists say that it is possible to calculate the Hubble constant in a completely new way.
“The Hubble constant tells you the size and the age of the universe; it’s been a holy grail since the birth of cosmology. Calculating this with gravitational waves could give us an entirely new perspective on the universe,” said Holz in a statement. “The question is: When does it become game-changing for cosmology?”
The ripples of an expanding universe
The existence of gravitational waves, which were first predicted by Einstein’s Theory of General Relativity about a hundred years ago, was confirmed only recently. The event was recorded by the Laser Interferometer Gravitational-Wave Observatory (LIGO), whose founders were awarded the Nobel Prize in Physics for this achievement.
Gravity waves are essentially ripples in the fabric of spacetime which are generated by interactions between very massive accelerating cosmic objects, such as neutron stars or black holes. In many ways, gravity waves are not all that different from the waves generated when a stone is thrown into a pond.
LIGO was founded in 1992, so it took them 25 years to prove the existence of gravity waves, because detecting a gravity wave relies on extreme precision. Scientists had to measure a distance change 1,000 times smaller than the width of a proton using interferometers, which are basically mirrors placed 4 kilometers apart.
The first detection of a gravitational wave occurred on September 14, 2015, and was produced by the collision of two black holes — an event which was announced on February 11, 2016. Since then, scientists have observed a number of other events, including gravitational waves produced by the merger of two neutron stars. This was the first time signals originating from the same source were detected with both traditional telescopes that detect light, and gravitational wave detectors that sense wrinkles in the fabric of space-time.
The remarkable discovery also enabled the University of Chicago team of researchers to devise their novel method. By measuring the signal emitted by colliding stars, scientists can get a signature of their mass and energy. When they compare this reading with the strength of the gravitational waves, they can infer how far away it is.
Holz says that this method involves fewer assumptions about the universe, which would make the computer Hubble constant more precise. The only important variable that affects the precision of the measurement is how often scientists can record gravitational wave. Holz and colleagues estimate that detecting 25 readings from neutron star collisions could enable them to measure the expansion of the universe with a 3% accuracy. At 200 readings, the error narrows down to only 1% — and this all might take only a couple of years, the authors reported in the journal Nature.
A precise value for the Hubble constant could tell us whether the nature of gravity has changed over time or could even shed light on dark energy, the force responsible for the expansion of the universe. Also importantly, scientists could arrive at a much more precise value for the age of the universe.
“With the collision we saw last year, we got lucky—it was close to us, so it was relatively easy to find and analyze,” said Maya Fishbach, a UChicago graduate student and the other author on the paper. “Future detections will be much farther away, but once we get the next generation of telescopes, we should be able to find counterparts for these distant detections as well.”
“It’s only going to get more interesting from here,” Holz said.
Dark energy is the mysterious force that drives the Universe’s expansion at an ever increased pace. Probing and understanding this force is thus imperative for astronomers’ and cosmologists’ efforts of peering through the Universe’s secrets. Recently, a new massive project set on probing the nature of dark energy was launched, called the Dark Energy Survey (DES), and its future findings are already regarded in great promise.
The Dark Energy Camera photographs galaxies from its perch on the Blanco telescope in Chile. (c) Reidar Hahn/Fermilab
DES’ centerpiece is its 570-megapixel digital camera (pictured), capable of imaging 300 million galaxies over one-eighth of the clear night sky from up atop the the 4-metre Blanco telescope at the Cerro Tololo Inter-American Observatory in the Chilean Andes. Galaxies and galaxy clusters are its main target since these are the brightest objects in the night’s sky and the camera’s high resolution capability can be most efficient. You see, to probe dark energy, the astronomers will measure weak gravitational lensing – the phenomenon in which incoming light from distant galaxies is skewed and distorted by matter between the galaxies and the point of observation (Earth).
Although measuring weak lensing is difficult, the researchers are confident this can be achieved. How is this important? Well, by measuring the degree of lensing, astronomers can infer and map the matter in between galaxies and Earth. This allow them in turn to create a three-dimensional web that can reveal the fingerprints of dark energy through time. A similar effort to DES has already been operation for quite a while – the Japanese Hyper Suprime-Cam in Hawaii, which relies on an even more detailed, 870-megapixel camera. Although Hyper Suprime can image fainter galaxies, DES can cover a wider patch of the sky. Together, the researchers hope these two projects will help paint a more accurate picture.
Besides weak lensing, DES will also be used to count galaxy clusters and measure their distance away from Earth, and spot distant supernovae, whose otherwise reference light is dimmed as the universe expands. The latter are of great importance since this is how the theory of an accelerating Universe was formed, netting the authors of the paper a Nobel prize in 2011.