How do you find dark energy when you don’t even know what you’re looking for?
Any difficult search is inevitably described as “like finding a needle in a haystack.” But the search for dark energy is just the opposite: We — everything in the universe made of “normal” matter and energy — are the needle, surrounded by the “haystack” of dark energy. To complicate things even more, this search is like trying to find a haystack when you aren’t sure if it’s a stack or even if it’s made of hay.
Since scientists don’t know what dark energy is, they aren’t searching for it directly — at least not yet. Instead, they study its effect: the accelerating expansion of the universe, which has provided much of the evidence of dark energy’s existence. The way in which the universe is accelerating, and changes in the acceleration over time, help scientists whittle down the list of possible explanations, and may even provide the answer.
The searches use three basic techniques
Plot the distances and velocities of the exploding stars known as supernovae.
Look at the way galaxies are distributed in the early universe to find patterns imprinted in the Big Bang.
Probe the “clumpiness” of the universe at different epochs by studying the shapes of millions of galaxies.
These studies probe the universe at different times, from a few hundred thousand years after the Big Bang to the modern epoch, providing a complete picture of how the acceleration has changed over the eons.
Supernovae illuminate the expanding universe
Astronomers discovered dark energy when they noticed that some exploding stars, called supernovae, were fainter than expected. The finding showed that the universe is expanding faster as it ages.
Several researchers use this same technique to probe larger areas of the universe over a greater span of time — from the modern day back to perhaps one or two billion years after the Big Bang.
The key to this technique is a type of exploding star called a Type Ia supernova — the complete destruction of a white dwarf star.
Such an explosion is visible across great cosmic distances, making Type Ia supernovae good “standard candles” for measuring the scale of the universe. That’s because Type Ia supernovae brighten and fade in a predictable way, so measuring how long it takes one of them to brighten and then fade reveals its true brightness. By comparing a star’s true brightness to how bright it appears, astronomers can find its distance.
Astronomers then measure how fast the star is moving away from Earth by measuring its redshift — a stretching of its light waves caused by the expansion of the universe itself.
Teams of astronomers are using this technique to study many more supernovae in different regions of the universe. Some use small, automated telescopes to scan large regions of the sky on different nights. By comparing images on different nights, they can detect supernovae, which flare to life in just a few hours. Once they spot a supernova, astronomers can then monitor it with larger telescopes to plot its lightcurve — the way it brightens and fades — and its spectrum.
Each supernova is at a different distance from Earth, which means we see it at a different time. A supernova that is one billion light-years away, for example, actually exploded one billion years ago, so its light gives us a view of the universe as it was one billion years ago. Supernova surveys find exploding stars at many different times — a span of several billion years.
These observations allow astronomers to determine how fast the universe was expanding at different times in its history. Each dark-energy theory makes different predictions about how the expansion rate is changing, so actually measuring that rate will strengthen or weaken different ideas.
Ripples from the primordial fireball tell dark energy’s secrets
Disturbances in the dense soup of the early universe set off sound waves, like the ripples when you throw a stone into a pond.
Sci-fi movies notwithstanding, you can’t hear sounds in the vacuum of space. But you should be able to “see” the sounds of the early universe preserved in the way galaxies are distributed through space. That pattern should reveal important details about the epoch shortly after the Big Bang — including the composition of dark energy.
Until about 400,000 years after the Big Bang, the universe was a dense, hot cauldron of particles of matter and energy. Disturbances in this dense soup set off sound waves, like the ripples when you throw a stone into a pond. These sound waves helped matter begin to clump together, forming the first structure in the universe. We see the result of this clumping in the cosmic microwave background radiation — the “afterglow” of the Big Bang.
But we should also see it in the way galaxies are distributed at different times in the history of the universe. The ripples in the early universe established a basic “yardstick” for the distribution of matter. As the universe expands, the yardstick expands with it. By measuring the size of the yardstick at different times in the history of the universe, astronomers can plot how the rate of expansion of the universe has changed.
The yardstick was preserved as the universe expanded and cooled further and the first galaxies took shape, but it’s not easy to detect. A look at even a tiny region of the sky reveals millions of galaxies distributed through space and time — some of them are quite close, which means we see them as they appeared just a few million years ago, while others are billions of light-years away, so we see them as they appeared when the universe was young.
So, astronomers must measure millions of galaxies, isolate them by distance (which means at different ages of the universe), then analyze the patterns at different epochs. The HETDEX project, for example, will produce a 3-D map of at least one million galaxies that are roughly 9 billion to 11 billion light-years away, which means we see them as they looked when the universe was as little as 20 percent of its current age.
From that map, astronomers will measure distances between individual galaxies to look for patterns in their arrangement. They should find that at different times in the history of the universe, galaxies “prefer” a particular distance from one another — like the crests of the waves on a pond. Astronomers then compare this distance to the yardstick imprinted in the early universe to determine how the expansion rate of the universe has changed over the eons.
Measuring the expansion rate at different times shows the “strength” of dark energy at different epochs. Different models of dark energy predict different changes in the expansion rate, so measuring the length of a basic cosmic yardstick — the imprint of ancient sound waves — may reveal the nature of dark energy.
Light twists its way through a lumpy universe
As a light ray travels from a distant galaxy to Earth, it gets pushed around a little. The gravity of intervening objects, such as galaxies, galaxy clusters, or clouds of dark matter, “warps” the space around it. This warp deflects the path of any passing light ray. This effect is called gravitational lensing, and it could provide important clues to the nature of dark energy.
There are two types of gravitational lensing.
Strong lensing clearly distorts the view of distant galaxies, sometimes producing several images of a single galaxy. Strong lensing occurs when a distant galaxy lines up directly behind a massive galaxy or galaxy cluster, which exerts a strong gravitational pull.
Weak lensing occurs when the light from a distant galaxy passes a good distance from a massive galaxy, galaxy cluster, or dark-matter concentration, or closer to less-massive objects. It produces a slight distortion in the shape of a distant galaxy. The effect is so subtle that you can’t notice a difference just by looking at the galaxy. Instead, astronomers must analyze the shapes of millions of galaxies to search for patterns. These patterns will allow them to produce three-dimensional maps of the distribution of matter throughout the universe.
These maps will clearly show the distribution of dark matter. But they also will help scientists understand the nature of dark energy.
As with other search methods, weak lensing allows astronomers to probe much of the history of the universe, from shortly after the Big Bang to today. That will show how the distribution of matter has changed over time — a function that is controlled in part by dark energy.
Dark energy causes space itself to expand, so the universe gets bigger. As it does so, matter becomes more thinly distributed and the light that provides the weak lensing has to travel a greater distance across the universe. Since astronomers know how much matter the universe contains, measuring both how widely it is spread out and the distance traveled at different times will show how the universe has expanded. Different models for dark energy predict different expansion histories, so determining how the universe has expanded will help select the correct explanation.
At the same time, dark matter would have required that galaxies clump together to form large-scale structures like clusters and superclusters early in the universe, when matter was more densely packed and gravity overwhelmed dark energy’s repulsive force. As the universe grew, dark energy became more dominant, preventing galaxies from forming large clusters. The weak-lensing method will show when galaxies stopped clumping together, indicating how and when dark energy exerted its dominance over gravity.
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