by Michio Kaku
Ultimately, the final resolution to all these paradoxes may come when the complete theory of quantum gravity is formulated. For example, perhaps when you enter a time machine, your time line might split, and you create a parallel quantum universe. Let’s say you go back in time and save Abraham Lincoln from being assassinated at Ford’s Theatre. Then perhaps you have saved Abraham Lincoln but in a parallel universe. Hence, the Abraham Lincoln in your original universe did die, and nothing will change that. But the universe has split into two universes, and you have saved President Lincoln in a parallel universe.
So, by assuming the time line can split into a parallel universe, all the paradoxes of time travel can be resolved.
The question of time travel can be definitively answered only when we can calculate the graviton quantum corrections, which we have ignored so far. Physicists have applied the quantum theory to stars and wormholes, but the key is to apply the quantum theory to gravity itself via gravitons, which requires a theory of everything.
This discussion raises interesting questions. Can quantum mechanics fully explain the nature of the Big Bang? Can quantum mechanics applied to gravity answer one of the great questions of science: What happened before the Big Bang?
How Was the Universe Created?
Where did the universe come from? What set the universe into motion? These are perhaps some of the greatest questions in all of theology and science, the subject of endless speculation.
The ancient Egyptians believed that the universe started as a cosmic egg floating in the Nile. Some Polynesians believed that the universe started with a cosmic coconut. Christians believe the universe was set into motion when God said, “Let there be light!”
The origin of the universe has also fascinated physicists, especially when Newton gave us a compelling theory of gravity. But when Newton tried to apply his theory to the universe we see around us, he encountered problems.
In 1692, he received a disturbing letter from clergyman Richard Bentley. In the letter, Bentley asked Newton to explain a hidden, possibly damaging flaw in his theory. If the universe is finite, and if gravity is always attractive, rather than repulsive, then eventually all the stars in the universe will be attracted to one another. In fact, with enough time, they will all coalesce into a single, gigantic star. So a finite universe should be unstable, and must eventually collapse. Since this does not happen, there must be a flaw in Newton’s theory.
Next, he argued that Newton’s laws predicted an unstable universe even if the universe were infinite. In an infinite universe, with an infinite number of stars, the sum of all forces tugging on a star from the left and right would also be infinite. Hence, these infinite forces would eventually tear the stars apart, and hence all stars would disintegrate.
Newton was disturbed by this letter, because he had not considered applying his theory to the entire universe. Eventually, Newton came up with a clever but incomplete answer to this question.
Yes, he admitted, if gravity is always attractive, and never repulsive, then the stars in the universe might be unstable. But there was a loophole in this argument. Assume that the universe is, on average, totally uniform and infinite in all directions. In such a static universe, all the forces of gravity cancel one another out, and the universe becomes stable once again. Given any star, the forces of gravity acting on it from all the distant stars in different directions eventually sum to zero, and hence the universe does not collapse.
Although this was a clever solution to this problem, Newton realized there was still a potential flaw to his solution. The universe might be uniform on average, but it cannot be exactly uniform at all points, so there must be tiny deviations. Like a house of cards, it appears to be stable, but the tiniest flaw will cause the entire structure to collapse. So Newton was clever enough to realize that a uniform infinite universe was indeed stable but was always teetering on the edge of collapse. In other words, the cancellation of infinite forces must be infinitely precise or else the universe will either collapse or be ripped apart.
Thus, Newton’s final conclusion was that the universe was infinite and uniform on average, but occasionally God has to tweak the stars in the universe, so they do not collapse under gravity.
Why Is the Night Sky Black?
But this raised another problem. If we start with a universe that is infinite and uniform, then everywhere we look into space our gaze will eventually hit a star. But since there are an infinite number of stars, there must be an infinite amount of light entering our eyes from all directions.
The night sky should be white, not black. This is called Olbers’ paradox.
Some of the greatest minds in history have tried to tackle this sticky question. Kepler, for example, dismissed the paradox by claiming that the universe was finite, and hence there is no paradox. Others have theorized that dust clouds have obscured starlight. (But this cannot explain the paradox, because, in an infinite amount of time, the dust clouds begin to heat up and then emit blackbody radiation, similar to a star. So the universe becomes white again.)
The final answer was actually given by Edgar Allan Poe in 1848. Being an amateur astronomer, he was fascinated by the paradox and said that the night sky is black because, if we travel back in time far enough, we eventually encounter a cutoff—that is, a beginning to the universe. In other words, the night sky is black because the universe has a finite age. We do not receive light from the infinite past, which would make the night sky white, because the universe never had an infinite past. This means that telescopes peering at the farthest stars will eventually reach the blackness of the Big Bang itself.
So it is truly amazing that by pure thought, without doing any experiments whatsoever, one can conclude that the universe must have had a beginning.
General Relativity and the Universe
Einstein had to confront these puzzling paradoxes when he formulated general relativity in 1915.
Back in the 1920s, when Einstein first began to apply his theory to the universe itself, astronomers told him that the universe was static, neither expanding nor contracting. But Einstein found something disturbing in his equations. When he tried to solve them, the equations told him that the universe was dynamic, either expanding or contracting. (He did not realize this at the time, but this was the solution to the question asked by Richard Bentley. The universe did not collapse under gravity because the universe was expanding, overcoming the tendency to collapse.)
In order to find a static universe, Einstein was forced to add a fudge factor (called the cosmological constant) into his equations. By adjusting its value by hand, he could cancel out the expansion or contraction of the universe.
Later, in 1929, astronomer Edwin Hubble, by using the giant Mount Wilson Observatory telescope in California, was able to make a startling discovery. The universe was expanding after all, just as Einstein’s equations originally predicted. He made this historic discovery by analyzing the Doppler shift of distant galaxies. (When a star moves away from us, the wavelength of its light is stretched so it turns slightly reddish. When the star moves toward us, the wavelength is compressed, so it turns slightly blueish. By carefully analyzing the galaxies, Hubble found that, on average, the galaxies were redshifted and so moving away from us. The universe is expanding.)
In 1931, Einstein visited the Mount Wilson Observatory and met with Hubble. When Einstein was told that the cosmological constant was unnecessary, that the universe was expanding after all, he admitted that the cosmological constant was his “greatest blunder.” (Actually, as we shall see, the cosmological constant has made a comeback in recent years, so even his blunders apparently open entirely new areas of scientific investigation.)
It was also possible to take this result one step further and calculate the age of the universe. Since Hubble could calculate the rate at which the galaxies were moving away, it should be possible to �
��run the videotape backward,” and calculate for how long this expansion has taken place. The original answer for the age of the universe came out as 1.8 billion years (which was an embarrassment, since the Earth was known to be older than that—4.6 billion years. But fortunately, the latest satellite data from the Planck satellite puts the age of the universe at 13.8 billion years).
Quantum Afterglow of the Big Bang
The next revolution in cosmology took place when physicists began to apply the quantum theory to the Big Bang. Russian physicist George Gamow pondered whether, if the universe started off as a gigantic, superhot explosion, some of that heat would survive today. If we apply the quantum theory to the Big Bang, then the original fireball must have been a quantum blackbody radiator. Since the properties of a blackbody radiator are well-known, it should be possible to calculate the radiation that is the afterglow, or echo, of the Big Bang.
Using the primitive experiments available, in 1948 Gamow and his colleagues Ralph Alpher and Robert Herman calculated that the temperature of the afterglow of the Big Bang should today be around five degrees above absolute zero. (The actual number is 2.73 K.) This is the temperature of the universe after it has cooled for billions of years.
This prediction was verified in 1964 when Arno Penzias and Robert Wilson used the giant Holmdel radio telescope to detect this residual radiation in space. (At first, they thought that this background radiation was due to a defect in their apparatus. According to legend, they realized their mistake when they gave a talk at Princeton, and someone in the audience said, “Either you have detected bird shit, or the creation of the universe.” To test this, they had to carefully scrape all the pigeon droppings off the radio telescope.)
Today, this microwave background radiation is perhaps the most persuasive and convincing evidence for the Big Bang. As predicted, recent satellite photographs of the background radiation show a uniform fireball of energy evenly distributed around the universe. (When you hear static on a radio, some of that static actually comes from the Big Bang.)
In fact, these satellite photographs are now so precise that it is possible to detect tiny, minuscule ripples in the background radiation due to the quantum uncertainty principle. At the instant of creation, there should have been quantum fluctuations that caused these ripples. A perfectly smooth Big Bang would have violated the uncertainty principle. These ripples eventually expanded with the Big Bang to create the galaxies we see all around us. (In fact, if our satellites had not detected these quantum ripples in the background radiation, their absence would have destroyed the hope of applying the quantum theory to the universe.)
This gives us a remarkable new picture of the quantum theory. The very fact that we exist in the Milky Way galaxy, in the presence of billions of other galaxies, is due to these tiny quantum fluctuations in the original Big Bang. Billions of years ago, everything you see around you was a tiny dot in this background radiation.
The next step in the application of the quantum theory to gravity was when the lessons of the quantum theory and the Standard Model were applied to general relativity.
Inflation
Buoyed by the success of the Standard Model in the 1970s, physicists Alan Guth and Andrei Linde asked themselves: Could the lessons learned from the Standard Model and the quantum theory be applied directly to the Big Bang?
This was a novel question, since applying the Standard Model to cosmology was still unexplored. Guth noticed that there were two puzzling aspects of the universe that could not be explained by the Big Bang as they conceived of it.
First, there is the flatness problem. Einstein’s theory states that the fabric of space-time should have a slight curvature. But when analyzing the curvature of the universe, it seems to be much flatter than predicted by Einstein’s theory. In fact, the universe appears to be perfectly flat, to within experimental error.
Second, it is much more uniform than it should be. In the Big Bang, there should have been irregularities and imperfections in the original fireball. Instead, the universe appears to be quite uniform, no matter where we gaze into the heavens.
Both of these paradoxes can be solved by invoking the quantum theory, with a phenomenon Guth called inflation. First, according to this picture, the universe underwent a turbocharged expansion, much faster than originally postulated for the Big Bang. This fantastic expansion basically flattened the universe and eliminated whatever curvature the original universe had.
Second, the original universe might have been irregular, but a tiny piece of that original universe was uniform and was inflated to enormous size. Hence, that would explain why the universe seems to be so uniform today, because we are descended from a tiny, uniform piece of the larger fireball that gave us the Big Bang.
The implications of inflation are far-reaching. It means that the visible universe that we see around us is actually a tiny, infinitesimal piece of a much larger universe, one that we will never see because it is so far away.
But what caused inflation in the first place? What set it in motion? Why did the universe expand at all? Guth then took some inspiration from the Standard Model. In the quantum theory, you start with a symmetry, and then you break it with the Higgs boson to get the universe that we see all around us. Similarly, Guth then theorized that maybe there was a new type of Higgs boson (called the inflaton) that made inflation possible. As with the original Higgs boson, the universe started out in the false vacuum that gave us the era of rapid inflation. But then quantum bubbles occurred within the inflaton field. Inside the bubble, the true vacuum emerged, where the inflation had stopped. Our universe emerged as one of these bubbles. The universe slowed down within the bubble, giving us the present-day expansion.
So far, inflation seems to fit the astronomical data. It is currently the leading theory. But it has unexpected consequences. If we invoke the quantum theory, it means that the Big Bang can happen again and again. New universes may be being born out of our universe all the time.
This means that our universe is actually a single bubble in a bubble bath of universes. This creates a multiverse of parallel universes. This still leaves open a nagging question: What was driving inflation in the first place? That, as we shall see in the next chapter, requires an even more advanced theory, a theory of everything.
Runaway Universe
General relativity not only gives us unprecedented insight into the beginning of the universe, it also gives us a picture of its ultimate fate. Ancient religions, of course, have given us stark images of the end of time. The ancient Vikings believed the world will end in Ragnarok, or the Twilight of the Gods, when a gigantic snowstorm will engulf the entire planet, and the gods will fight the final battle against their celestial enemies. To Christians, the Book of Revelation foretells disasters, cataclysms, and the coming of the Four Horsemen of the Apocalypse, which precede the Second Coming.
But to a physicist, there are traditionally two ways in which everything will end. If the density of the universe is low, then there is not enough gravity from the stars and galaxies to reverse the cosmic expansion, and the universe will expand forever and slowly die in the Big Freeze. The stars will eventually use up all their nuclear fuel, the sky will turn black, and even black holes will evaporate. The universe will end in a lifeless, supercold sea of drifting subatomic particles.
If the universe is sufficiently dense, then the gravity of the stars and galaxies might be enough to reverse the cosmic expansion. Then the stars and galaxies will eventually collapse into the Big Crunch, when temperatures soar and devour all life in the universe. (Some physicists have even conjectured that the universe may then bounce back in another Big Bang, creating an oscillating universe.)
But in 1998, astronomers made a stunning announcement that overturned many of our cherished beliefs and forced us to revise our textbooks. By analyzing distant supernovae throughout the universe, they found that t
he universe was not slowing down in its expansion, as previously thought, but actually speeding up. In fact, it was entering a runaway mode.
They had to revise the two previous scenarios, and a new theory emerged. Perhaps the universe will die in something called the Big Rip, in which the expansion of the universe accelerates to blinding speed. The universe will expand so quickly that the night sky will become totally black (since light cannot reach us from neighboring stars) and everything approaches absolute zero.
At that temperature, life cannot exist. Even the molecules in outer space lose their energy.
What might be driving this runaway expansion is something that was once discarded by Einstein in the 1920s, the cosmological constant, the energy of the vacuum, now called dark energy. Surprisingly, the amount of dark energy in the universe is enormous. More than 68.3 percent of all matter and energy in the universe is in this mysterious form. (Collectively, dark energy and dark matter comprise most of the matter/energy, but they are two distinct entities and should not be confused with each other.)
Ironically, this cannot be explained by any known theory. If one tries to blindly calculate the amount of dark energy in the universe (using the assumptions of relativity and the quantum theory), we find a value that is 10120 times larger than the actual value! (That is the number 1 followed by 120 zeros.)
This is the largest mismatch in the entire history of science. The stakes could not be higher: the ultimate fate of the universe itself is hanging in the balance.
This could tell us how the universe itself will die.
Wanted: The Graviton
