In 1905, a 26-year-old Albert Einstein changed physics forever when he outlined his theory of special relativity. This theory outlined the relationship between space and time and is founded on two fundamental assumptions: the laws of physics are the same for all non-accelerating observers, and the speed of light in a vacuum is always the same.
Over the last century, Einstein's theories of relativity (both special and general) have withstood the trials of experimental verification and been used to explain a number of physical processes, including the origins of our universe. But in the late 1990s, a handful of physicists challenged one of the fundamental assumptions underlying Einstein's theory of special relativity: Instead of the speed of light being constant, they proposed that light was faster in the early universe than it is now.
This theory of the variable speed of light was—and still is—controversial. But according to a new paper published in November in the physics journal Physical Review D, it could be experimentally tested in the near future. If the experiments validate the theory, it means that the laws of nature weren't always the same as what we experience today and would require a serious revision of Einstein's theory of gravity.
"The whole of physics is predicated on the constancy of the speed of light," Joao Magueijo, a cosmologist at Imperial College London and pioneer of the theory of variable light speed, told Motherboard. "So we had to find ways to change the speed of light without wrecking the whole thing too much."
"The whole of physics is predicated on the constancy of the speed of light."
According to Magueijo, the variable speed of light (VSL) theory emerged as a solution to a longstanding inconsistency in cosmology known as "the horizon problem" which arises when the speed of light is considered to be a constant.
If light has an invariable speed limit, then that means that since the Big Bang it could only have traveled approximately 13.7 billion light years, because approximately 13.7 billion years have elapsed since the Big Bang. The distance that light is able to travel since the Big Bang creates the 'horizon' of the visible universe—this is about 47 billion light years (although light has only been traveling for 13.7 billion years, this number takes into account the expansion of space that is occurring as light is traveling).
So imagine sitting in the center of a sphere (the universe) with a diameter of 47 billion light years. The edge of this sphere, aka the horizon of the universe, is the cosmic microwave background (CMB)—radiation from about 400,000 years after the Big Bang and our earliest snapshot of the universe—and no matter where you are in the universe, when you observe the CMB today it is 13.7 billion light years distant.
Here's where the problem arises: although any point in the universe is always 13.7 billion light years from the cosmic microwave background, the distance separating one side of the horizon of the cosmic microwave background from the other (let's call this the "diameter" of the universe) is approximately 27.4 billion light years. In other words, the universe is too large to have allowed light to travel from one end of the other during its existence, which is necessary to account for the homogeneity observed in the CMB.
When cosmologists observe the cosmic microwave background it is remarkably uniform: its temperature is approximately -270 C no matter where it is measured with minuscule variance (one part in 100,000). Yet if light, the fastest "thing" in the universe, isn't able to travel from one side of the universe to the other over the course of the universe's entire existence, this uniformity that is observed in the CMB would be impossible.
To understand why this is the case, imagine a bathtub with a faucet at either end, one spigot producing cold water, the other would produce hot water. If you turn both of these faucets off, eventually the water in the bathtub will reach a uniform temperature as the hot and cold water mix. But if while the faucets are running you stretch the tub out in every direction so fast that the hot and cold water will never meet, one side of the tub will always be way hotter than the other side instead of a single uniform temperature.
This is what happened during the Big Bang, except that rather than seeing parts of the early universe in the CMB that are way hotter or cooler than other parts, it's perfectly uniform. So what gives?
The most widely accepted resolution to the horizon problem is called inflation, which basically states that the uniformity we observe in the CMB occurred while the universe was still incredibly small and dense, and it maintained this uniformity while it expanded. In this example, the hot and cold bath water reached a uniform temperature before the bathtub started its crazy fast expansion in every direction.
Although this inflationary theory preserves a constant speed of light, it also requires accepting the existence of an "inflation field," which only existed during a brief period of time in the early universe.
According to proponents of variable light speed however, this problem can be solved without recourse to inflation if the speed of light was significantly higher in the early universe. This would allow the distant edges of the universe to remain "connected" as the universe expanded and would account for the observed uniformity in the CMB.
Yet for theoretical physicists who prescribe to the inflationary model of the universe, allowing for variable light speed instead of constant light speed is a way of "flipping the sign" of a fundamental term in the theory of special relativity.
"In most cases, flipping such a sign is a recipe for certain disaster as the resulting theory would cease to be physically and internally consistent," David Marsh, a senior research fellow at the Center for Theoretical Cosmology who was not involved with the paper, told Motherboard. "Afshordi and Magueijo have addressed some of the challenges coming with this sign flip, but it appears that much work remains in establishing that the model is theoretically healthy. If that can be done, this model may have a host of far-reaching consequences also for the rest of physics beyond cosmology."
So just how much faster was light speed just after the Big Bang? According to Magueijo and his colleague Niayesh Afshordi, an associate professor of physics and astronomy at the University of Waterloo, the answer is "infinitely" faster.
The duo cite light speed as being at least 32 orders of magnitude faster than its currently accepted speed of 300 million meters per second—this is merely the lower bounds of the faster light speed, however. As you get closer to the Big Bang, the speed of light approaches infinity.
On this view, the speed of light was faster because the universe was incredibly hot at the beginning. According to Afshordi, their theory requires that the early universe was at least a toasty 10^28 degrees Celsius (to put this in perspective, the highest temperature we are capable of realizing on Earth is about 10^16 degrees Celsius, a full 12 orders of magnitude cooler).
As the universe expanded and cooled below this temperature, light underwent a phase shift—much like liquid water changes into ice once the temperature reaches a certain threshold—and arrived at the speed we know today: 300 million meters per second. Just like ice won't get more "icy" the colder the temperature gets, the speed of light has not been slowing down since it reached 300 million meters per second.
If Magueijo and Afshordi's theory of variable light speed is correct, then the speed of light decreased in a predictable way—which means with sensitive enough instruments, this light speed decay can be measured. And that's exactly what they did in their latest paper.
"Varying speed of light is going back to the foundations of physics and saying perhaps there are things beyond relativity."
According to Afshordi, galaxies and other structures in the universe were only possible due to fluctuations in the early universe's density. These density fluctuations are recorded in the cosmic microwave background as a "spectral index," which might be imagined as the "color" of the early universe. The neutral baseline of the spectral index is a value of 1, which would be a universe with the same magnitude of gravitational fluctuations on all scales. Above this value the universe is "blue" (representing shorter wavelength fluctuations) and below this value and the universe is "red"(representing longer wavelength fluctuations).
Although the inflationary model of the universe also would have a "red" spectral index, it is unable to calculate a precise value of the index and as a result the exact gravity fluctuations in the early universe. In their new paper, Magueijo and Afshordi pegged the spectral index at a value of 0.96478, just slightly red, which is two orders of magnitude more precise than current measurements of the spectral index (about 0.968).
Now that they've used the variable light speed theory to put a hard number on the spectral index, all that remains to be seen is whether increasingly sensitive experiments probing the CMB and distribution of galaxies will verify or overturn their theory. Both Magueijo and Afshordi expect these results to be available at some point in the decade. But Marsh and other physicists aren't so sure.
"Compared to inflation, Afshordi and Magueijo's model is at the present very complicated and poorly understood," Marsh said. "However, the understanding of inflation has developed over 35 years and there are still important open theoretical questions to address in that framework. It is certainly possible that given more time and research, the theoretical setting of this model will be much better understood and its predictions may appear more elegant."
If their theory is correct, it will overturn one of the main axiom's underlying Einstein's theory of special relativity and force physicists to reconsider the nature of gravity. According to Afshordi, however, it is more or less accepted in the physics community that Einstein's theory of gravity cannot be the whole story, and that a quantum theory of gravity will come to replace it. There are a number of competing quantum gravity theories, but if the variable light speed theory outlined in this paper is proven to be correct, it will significantly narrow the range of plausible theories of quantum gravity.
"If you really want to open up quantum gravity to observation, you're better off without this idea of inflation," said Magueijo. "Inflation leaves fundamental physics completely untouched [and] is a mechanism of insulating the observable universe from physics beyond relativity. Varying speed of light is going back to the foundations of physics and saying perhaps there are things beyond relativity. This is the best position to open up new ideas and new theories."
Correction: This story originally stated that the "horizon" of the visible universe was 13.7 billion light years, when in fact it is 47 billion light years. We've updated the reference and regret the error.
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