For the past two decades, much of cosmology has rested on the idea that the universe is expanding at an accelerated rate, driven by an unseen agent that is somehow both ubiquitous yet undetected called dark energy. However, a team of Oxford University scientists led by Subir Sarkar has challenged that idea, first reported in 1998 by two independent groups of physicists.
Working with a supernova database roughly ten times larger than the one available to those physicists, Sarkar and his team have concluded that there is little evidence to support the idea of accelerated expansion and that the universe seems instead to be expanding at a constant velocity.
While Sarkar and his team did not disprove the accelerated-rate theory, they claim their statistical analysis of the supernova database calls the theory into question, saying that dark energy might simply be a statistical error — the consequence of an antiquated theoretical model. The researchers acknowledged that more work must be done to corroborate their findings, but they hope their research will challenge astronomers “to investigate more nuanced cosmological models” and retain a healthy sense of skepticism before accepting scientific claims as fact.
Ironically, many physicists remain skeptical of the paper’s claims despite the wide attention it has received. In fact, only recently a team of scientists led by Nobel Prize winner Adam Riess completed a 10-year study on the same topic, and they concluded instead that the universe is expanding 5 to 9 times faster than previously believed.
Sarkar and his team are not the first to challenge the accelerated-rate theory. In 2010, astronomer Wun-Ti Shu of the National Tsing Hua University in Taiwan proposed an innovative model of the cosmos that eliminates the need for dark energy and the Big Bang; the theory, which states that time converts into space as the universe expands and vice versa as it contracts, holds that the universe has no beginning or end, but rather that it alternates infinitely between periods of expansion and contraction.
Similarly, in 2013 a theoretical physicist named Christof Wetterich put forth a model which claims that the universe is not expanding at all, but that the mass of everything has simply been increasing. Like Shu’s theory, Wetterich’s does away with dark energy and helps explain some of the observations that first inspired the accelerated-rate theory. But like every existing model, it falls short of explaining all the cosmic phenomena scientists have observed over the years, especially one important phenomenon called the Cosmic Microwave Background (CMB).
Ultimately, astronomers will not be abandoning the accelerated-rate theory anytime soon. The current model of the Big Bang is very popular among scientists, and the idea of a temporally infinite universe — one without beginning or end — was all but abandoned shortly after the discovery that there were galaxies outside of our own and that they were moving away from us.
The Model of an Expanding Universe
The modern concept of an expanding universe is almost 100 years old.
In the early 1920s, the prevailing idea was that the Milky Way was all there was. Astronomers believed our galaxy contained all the stars and nebulae visible in the night sky, and while some scientists had challenged that idea, claiming instead that those cloudy spiral structures were in fact “island universes” outside of our own, they could not yet accurately calculate their distances, and the matter remained up for debate.
In the mid-1920s, astronomer Edwin Hubble helped put this so-called Great Debate to rest.
Using new technology and building on the work of scientists who had come before him, Hubble was able to measure with great accuracy the distances between earth and certain pulsating stars in what we now call the Andromeda Galaxy. These pulsating stars, called Cepheid variables, brighten and dim at very regular intervals. When observed over time, they can be used to calculate distances in space.
After observing these stars, Hubble found that they were too far away to be part of the Milky Way and concluded that the nebula that housed them, previously thought to exist within our galaxy, was in fact a galaxy all on its own.
Hubble’s discovery helped give weight to the existing “island universe” theory, and it drastically altered our perception of the scale of the universe.
A few years later, in 1929, Hubble would make another discovery that would once again turn the prevailing ideas about astronomy upside-down.
After studying the Doppler shifts of several galaxies through the Hooker telescope, one of the largest in the world at the time, Hubble found that with some exceptions, the vast majority of galaxies were moving away from earth at speeds relative to their respective distances — that is, the farther a galaxy was, the faster it was moving away.
Much like the siren of an ambulance, which sounds higher-pitched as the vehicle moves toward a listener and lower-pitched as it moves farther away, the light we observe from distant galaxies appears one way when the galaxy is moving toward us and another way when the galaxy is moving away.
This phenomenon is called the Doppler effect.
When an ambulance moves toward us, the siren’s sound waves “bunch up,” which means their wavelengths — roughly, the distances between each wave — become shorter. This causes the siren to sound more high-pitched as it gets closer to us. Similarly, when a galaxy moves toward us, the light’s wavelength decreases, making the light look more blue, as blue light has a shorter wavelength than red light.
And just as an ambulance sounds deeper as it moves away due to its sound waves’ being more “spaced out,” the light from distant galaxies looks redder as the galaxies move away, since red light has a longer wavelength than blue light. (In fact, red light has the longest wavelength of all the light our eyes can see, which is just a tiny sliver of all the light that exists in the universe.)
So, building on the work of astronomer Vesto Slipher, Hubble was able to determine the velocities of nearby galaxies by measuring this redshift. While a few galaxies, like Andromeda, appeared blueshifted, which means they were moving toward Earth — indeed, it is estimated that the Andromeda galaxy will collide with our own galaxy in about 4 billion years — Hubble found that most galaxies exhibited redshift. In fact, the farther away a galaxy was, the more redshifted it appeared.
This relationship is known as Hubble’s Law, and to this day, the most popular interpretation of the data is that the universe is expanding.
The Big Bang Theory and the CMB
Once scientists learned that space itself was expanding in all directions and that matter wasn’t simply moving out into already pre-existing space, some of them reasoned that at some point in the past the universe must have been much smaller.
This theory was proposed in 1922 by Soviet scientist Alexander Friedmann and in 1927 by a priest named George Lemaître, who had also independently calculated the Hubble Law based on the redshifts of spiral nebulae. Lemaître proposed a model which held that the universe had begun with the explosion of a “primitive atom.” This model was adopted and developed by other scientists, and in the mid-20th century became known as the Big Bang theory.
During that time, there was one major model competing with the Big Bang theory. Spearheaded by English astronomer Fred Hoyle, the Steady State theory held that as the universe expanded, it simply created new matter, preventing the universe’s overall density from decreasing. According to this theory, the universe has no beginning or end, and its appearance remains largely unchanged over time.
There were various problems with the Steady State theory, but the biggest involved its inconsistency with observed phenomena. Scientists’ observations had shown that the universe wasn’t static, but that it had changed over time and was probably still changing, which the Steady State theory did not predict. The final “nail in the coffin” for the theory came in the mid-1960s with the discovery of the cosmic microwave background (CMB).
One of the biggest pieces of evidence for the Big Bang theory, the CMB is a remnant of the universe’s early expansion. It is the oldest light in the universe and can be found by searching far enough in any direction.
As scientists look deeper into space, they are actually looking into the past. When we see an object, our eyes capture the light from that object and send signals to our brains, which interpret the information and form an image in our minds. The light travels from its source — a lamp, a screen, the sun — to the object and then into our eyes. This travel takes time as the light must first leave the object, travel through space, and reach our eyes.
So when we see something, we are not seeing it just as it is; we are seeing it as it was when its light first began its voyage to our eyes.
For objects on earth, this fact is relatively insignificant. Light travels at about 300 million meters per second. As far as our brains are concerned, we might as well be seeing objects close to earth just as they are. Even light from the sun, which is 93 million miles away, takes just eight minutes to reach us (which of course means that when we look at the sun, we are actually seeing it as it was eight minutes ago).
But when we consider cosmic distances, the finite speed of light takes on a deeper significance. Because light travels at a finite speed, as astronomers focus their telescopes on objects that are further and further out into the universe, they see older and older light — light that left those objects long ago and was still traveling through the vacuum of space from its original source when it reached their telescopes.
Therefore, when astronomers Arno Penzias and Robert Wilson began hearing an “inexplicable hum” through their radio telescope in 1964 and exhausted every possible explanation as to its source, they were surprised to find that they were actually hearing very old light — microwaves that had traveled an estimated 13.8 billion years through the vacuum of space to reach New Jersey, where the Holmdale Horn Antenna picked them up and helped the scientists form an idea of the very early stages of the universe.
A remnant of the universe just 380,000 years after the Big Bang, the CMB is almost perfectly isotropic, meaning it appears the same no matter what part of the sky one observes. This uniformity means that the CMB didn’t develop when the universe was anywhere close to its current size — otherwise, how could such vastly distant patches of it be so similar in composition? The likeliest explanation is that the CMB formed long ago, when the universe was much smaller and denser than it is today, before a period of rapid inflation.
No cosmological model other than the Big Bang theory, which predicted its existence, has managed to supply a convincing explanation for the cosmic microwave background. To this day it remains one of the strongest indications that the universe expanded from a much smaller and denser state, and before any cosmological model can be taken seriously by scientists, it must contend with the CMB.
Accelerated Expansion and Dark Energy
The most recent major developments in cosmology have to do once again with the expansion of the universe.
In 2011, physicists Saul Perlmutter, Adam Riess and Brian Schmidt won the Nobel Prize for their discovery of the universe’s accelerated expansion. The scientists were part of two separate teams that had set out years earlier to determine the future of the universe.
Scientists knew the universe was expanding, but they also knew that depending on the density of matter in the universe, that expansion would follow only a few possible progressions.
According to Albert Einstein’s theory of general relativity, all matter curves spacetime due to gravitational attraction. So scientists know that if all the matter in the universe were above a certain “critical density,” then the universe’s expansion would decelerate because of this gravitational attraction, and the whole universe would eventually collapse into a “big crunch.”
If, however, the amount of matter in the universe were below the critical density, then the universe would continue expanding.
In order to determine which outcome was more likely, the two teams of scientists observed type Ia supernovae. These are exploded white dwarfs that are very similar to each other and can be used to calculate cosmic distances accurately. Measuring these supernovae’s redshifts yields a measure of the size of the universe at the time when each particular star exploded. This allowed each team of scientists to graph the relationship between each supernova’s distance and its redshift.
After interpreting the data, the two groups of scientists independently concluded that contrary to popular belief, the universe’s rate of expansion was not decreasing, but increasing — the universe was expanding faster and faster as time went on.
This increasing rate of expansion meant that something had to be cancelling out the effect of gravity, which should have been pulling the matter in the universe closer and closer together.
This “something” was dark energy, so named because it was unseen and detected.
When cosmologist Michael Turner first coined the term in 1998, scientists weren’t sure what “dark energy” was. Hearkening back to Einstein’s theory of general relativity, some argued that it was simply a cosmological constant, something intrinsic in the fabric of space itself that caused it to expand. Others posited a variable field called quintessence which began repelling matter about 10 billion years ago. Still others believed that Einstein’s theory of general relativity had failed in predicting cosmological behavior on large scales and called for a modified version of the theory.
To this day, dark energy lives up to its name; scientists have been unable to detect its existence, and its ultimate nature remains unknown and unexplained.
The “discovery” of dark energy both furthers our understanding of the universe and marks its limits. It stands at the frontier of our cosmological knowledge and remains a testament to the ongoing nature of scientific endeavor, demonstrating that prevailing models change continually as scientists revise their ideas and question accepted beliefs based on new evidence.
Only time will tell how current cosmological models will evolve. As technology improves, astronomers are able to take more detailed measurements of the cosmos, leading them to reinforce existing theories or forcing them to invent new ones. In the future, astronomers may be able to detect dark energy and learn more about the forces that govern our universe.
Luckily, however, we have virtually all the time in the world to figure it out. Regardless of our theories and models, the universe will continue to evolve with or without our knowing how, and we can take comfort in knowing that our understanding of the universe will not determine its fate — we hope.