Many Worlds in One: The Search for Other Universes (5 page)
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Authors: Alex Vilenkin
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The process of helium formation began at about 3 minutes A.B. and was complete in less than a minute. The universe was still expanding at a furious rate, and both the density and the temperature were dropping very rapidly. But after the first few minutes packed with action, the pace of the cosmic drama was getting slower. Very little was happening to the matter particles, and the most notable change was in the radiation component of the fireball.
At the microscopic, quantum level, the radiation consists of photons, but macroscopically it can be pictured as consisting of electromagnetic waves—oscillating patterns of electric and magnetic energy. The higher the frequency of oscillation, the more energetic the constituent photons. Waves of different frequency produce different physical effects, and we know them under different names. Visible light corresponds only to a narrow range of frequencies in the full electromagnetic spectrum. Higher frequency waves are called X rays, and still higher-frequency waves are called gamma rays. Going down in frequency, we encounter microwaves and still lower, radio waves. All these waves propagate at the speed of light.
As the fireball temperature declined, the intensity of the radiation tapered, and its frequency gradually shifted from gamma rays to X rays and then to visible light. An important event occurred at 300,000 years A.B., when the temperature got low enough for electrons and nuclei to combine into atoms. Prior to that, electromagnetic waves were frequently scattered by charged electrons and nuclei. However, the interaction of radiation with electrically neutral atoms is very weak, so that once atoms were formed, the waves propagated freely through the universe, with practically no scattering at all. In other words, the universe suddenly became transparent to light.
What happens to the cosmic radiation after that? Not much, except the frequencies of the electromagnetic waves, and the corresponding temperature, keep declining with the expansion of the universe. At the time of neutral atom formation, the temperature of the radiation was 4000 degrees, somewhat below that at the surface of the Sun. If we had been there, and could
have tolerated such unhealthy conditions, we would have seen the universe ablaze with brilliant orange light. By the cosmic age of 600,000 years the light would change to red. At 1 million years, it would shift beyond the visible range, to the infrared part of the spectrum. So, as far as we’d have been concerned, the universe would have descended into complete darkness. Wave frequencies still continue to decline slowly: by the present time, corresponding to the cosmic age of about 14 billion years, they are down to the microwave range.
have tolerated such unhealthy conditions, we would have seen the universe ablaze with brilliant orange light. By the cosmic age of 600,000 years the light would change to red. At 1 million years, it would shift beyond the visible range, to the infrared part of the spectrum. So, as far as we’d have been concerned, the universe would have descended into complete darkness. Wave frequencies still continue to decline slowly: by the present time, corresponding to the cosmic age of about 14 billion years, they are down to the microwave range.
This history of the cosmic fireball was studied by Alpher and Herman, Gamow’s young collaborators. They followed it all the way to the present and reached a remarkable conclusion—that we should now be immersed in a sea of microwaves having the temperature of about 5 degrees kelvin.
Alpher and Herman’s work was published in 1948. You might think that it should have inspired a fair number of observers to search for cosmic microwaves. Indeed, the primeval radiation is a true smoking gun of the big bang, and its discovery should have a colossal significance. You might think also that, once the radiation is detected, a Nobel Prize would be awarded for its prediction. Alas, this is not how the events unfolded.
Odd as it may seem, the prediction of cosmic radiation was completely ignored for nearly two decades, until the radiation was accidentally discovered in 1965. Two radio astronomers, Arno Penzias and Robert Wilson, working at Bell Telephone Laboratories in New Jersey, detected a persistent noise in their sensitive radio antenna. The noise level could be characterized by a temperature of approximately 3 degrees kelvin and did not depend on the time of day or on the direction in which they pointed the antenna. Determined to get to the root of the problem, Penzias and Wilson painstakingly eliminated all possibilities they could think of. This included eviction of a pair of pigeons who were roosting in the antenna and removing what Penzias called the “white dielectric material” that was left after them. Nothing worked, however, and the origin of the noise remained enigmatic.
In the meantime, about 30 miles away, a group of physicists at Princeton University were busy building a radio detector of their own. The head of the group was Robert Dicke, an extraordinary physicist who was equally at
home in theory and experiment. Dicke realized that a hot early stage in the history of the universe should have left an afterglow, and he designed an antenna to search for it. When the Princeton group were ready to start their measurements, they learned about Penzias and Wilson’s predicament. They knew immediately that the bothersome noise that Penzias and Wilson were working so hard to eliminate was precisely the signal of cosmic microwaves that they were hoping to detect!
home in theory and experiment. Dicke realized that a hot early stage in the history of the universe should have left an afterglow, and he designed an antenna to search for it. When the Princeton group were ready to start their measurements, they learned about Penzias and Wilson’s predicament. They knew immediately that the bothersome noise that Penzias and Wilson were working so hard to eliminate was precisely the signal of cosmic microwaves that they were hoping to detect!
It is a fascinating question why the cosmic radiation had to be discovered by accident. Why had nobody listened to Alpher and Herman? Even if their papers were somehow overlooked, why did it take more than fifteen years for someone else to come up with the same prediction? After all, cosmic radiation was a direct consequence of Gamow’s hot big bang model.
One reason, it seems, was that physicists simply did not believe that the early universe was for real. “This is often the way it is in physics,” wrote the Nobel Prize-winning physicist Steven Weinberg. “Our mistake is not that we take our theories too seriously, but that we do not take them seriously enough.”
7
It did not help also that George Gamow was perhaps too colorful a character to be taken seriously by the physics community. A practical joker, composing “unprintable” limericks and often having one too many at the bar, he was surely not your typical physicist. Finally, by the mid-1950s neither Gamow nor Alpher and Herman were actively working on the big bang theory: Gamow was increasingly attracted to biology, where he suggested important insights into the genetic code, while Alpher and Herman left academia and moved on to careers in private industry. One cannot help thinking that lack of appreciation of their work must have played a role in those decisions. By the mid-1960s, when Penzias and Wilson were taking data from their antenna, the work of the Gamow group was all but forgotten.
7
It did not help also that George Gamow was perhaps too colorful a character to be taken seriously by the physics community. A practical joker, composing “unprintable” limericks and often having one too many at the bar, he was surely not your typical physicist. Finally, by the mid-1950s neither Gamow nor Alpher and Herman were actively working on the big bang theory: Gamow was increasingly attracted to biology, where he suggested important insights into the genetic code, while Alpher and Herman left academia and moved on to careers in private industry. One cannot help thinking that lack of appreciation of their work must have played a role in those decisions. By the mid-1960s, when Penzias and Wilson were taking data from their antenna, the work of the Gamow group was all but forgotten.
Penzias and Wilson measured the intensity of the radiation at a single frequency (to which their antenna was tuned), while the theory predicted that the radiation should be spread over a range of frequencies, with the intensity following a simple formula derived by Max Planck at the turn of the twentieth century. This prediction was spectacularly confirmed in 1990 by the Cosmic Background Explorer (COBE) satellite experiment, which found agreement with the Planck formula at the level of one part in 10,000.
The discovery of the cosmic radiation was no doubt an epoch-making
event in cosmology. This tangible relic of the primeval fireball gives us faith that we have not dreamed it all up, that there was indeed a hot early universe some 14 billion years ago. Penzias and Wilson received the 1978 Nobel Prize “for their discovery of cosmic microwave background radiation.” No prize for its theoretical prediction has ever been awarded.
event in cosmology. This tangible relic of the primeval fireball gives us faith that we have not dreamed it all up, that there was indeed a hot early universe some 14 billion years ago. Penzias and Wilson received the 1978 Nobel Prize “for their discovery of cosmic microwave background radiation.” No prize for its theoretical prediction has ever been awarded.
If the universe had started out perfectly homogeneous, then it would remain homogeneous to this day. The thin, uniform gas filling the universe would gradually be getting ever thinner, and the universe would remain permanently dark, with cosmic radiation slowly shifting to radio waves of lower and lower frequency. But one look at the night sky should be enough to convince you that our universe is not nearly so dull. The universe is lit up with shining stars that are scattered throughout space, forming a hierarchy of structures. The basic unit of this hierarchy is the galaxy, with a typical galaxy containing about 100 billion stars. Galaxies are grouped in clusters, which in turn form superclusters that extend up to a few hundred million light-years
i
—only 100 times smaller than the size of the currently observable universe.
i
—only 100 times smaller than the size of the currently observable universe.
Cosmologists attribute the origin of all these magnificent structures to tiny inhomogeneities that existed in the primeval fireball. Small inhomogeneities can grow into galaxies as a result of
gravitational instability
. Suppose some region of the universe is slightly denser than its surroundings. It will have stronger gravity and will attract more matter from the surrounding regions. As a result, the density contrast will keep growing, and a nearly homogeneous initial distribution of matter will evolve into a highly inhomogeneous one. Cosmologists believe that this is how galaxies, clusters, and superclusters were formed. According to the theory, the first galaxies were formed about 1 billion years after the big bang. Stars lit up the universe, and thus the cosmic dark age ended. The process of galaxy formation was complete in the not-so-distant past—at the cosmic age of about 10 billion years (“only” 4 billion years ago).
gravitational instability
. Suppose some region of the universe is slightly denser than its surroundings. It will have stronger gravity and will attract more matter from the surrounding regions. As a result, the density contrast will keep growing, and a nearly homogeneous initial distribution of matter will evolve into a highly inhomogeneous one. Cosmologists believe that this is how galaxies, clusters, and superclusters were formed. According to the theory, the first galaxies were formed about 1 billion years after the big bang. Stars lit up the universe, and thus the cosmic dark age ended. The process of galaxy formation was complete in the not-so-distant past—at the cosmic age of about 10 billion years (“only” 4 billion years ago).
You might think that this story is destined to remain just that—a story—since nobody was there to confirm it. But as I already emphasized, we see distant objects as they were a long time ago, when the light we now detect was emitted. Thus, by studying more distant galaxies, we go further back in time. The travel time of light from the most distant galaxies that we can observe is about 13 billion years, so we see them when the universe was a billion years old. Compared to the grand spirals we find nearby, these galaxies are small and irregular—a sign of their youth.
Still earlier epochs in the history of the universe can be observed through cosmic microwaves. These waves traveled without scattering for nearly 14 billion years, since the time when the universe became transparent to radiation. The regions where the waves were last scattered are now 40 billion light-years away
j
(not 14 billion light-years as one might think, since the universe was expanding in the meantime). Thus, the microwaves come to us from the surface of a gigantic sphere, 40 billion light-years in radius; it is called the
surface of last scattering
. Radiation emitted from regions of slightly higher density has to overcome stronger gravity and arrives to us with a slightly diminished intensity. As a result, denser regions look dimmer on the microwave sky. By mapping the radiation intensity from different directions in the sky, we can obtain an image of the universe at the epoch of last scattering, when it was only 300,000 years old.
j
(not 14 billion light-years as one might think, since the universe was expanding in the meantime). Thus, the microwaves come to us from the surface of a gigantic sphere, 40 billion light-years in radius; it is called the
surface of last scattering
. Radiation emitted from regions of slightly higher density has to overcome stronger gravity and arrives to us with a slightly diminished intensity. As a result, denser regions look dimmer on the microwave sky. By mapping the radiation intensity from different directions in the sky, we can obtain an image of the universe at the epoch of last scattering, when it was only 300,000 years old.
The first successful map of the microwave sky was made by the COBE team in 1992. A more detailed map, produced 10 years later by the WMAP satellite,
k
is shown in
Figure 4.2
. Darker shades of grey correspond to higher radiation intensity, but the difference in intensity between the lightest and darkest spots is only a few parts in 100,000. This means that at the time of last scattering the universe was almost perfectly homogeneous. All the glorious structures that we now see in the sky were then encoded in tiny amorphous ripples on the smooth cosmic background.
k
is shown in
Figure 4.2
. Darker shades of grey correspond to higher radiation intensity, but the difference in intensity between the lightest and darkest spots is only a few parts in 100,000. This means that at the time of last scattering the universe was almost perfectly homogeneous. All the glorious structures that we now see in the sky were then encoded in tiny amorphous ripples on the smooth cosmic background.
Figure 4.2
. Microwave sky as mapped by the WMAP satellite. (Courtesy of Max Tegmark)
. Microwave sky as mapped by the WMAP satellite. (Courtesy of Max Tegmark)
The picture in
Figure 4.3
illustrates the story of genesis as we have discussed it so far. This story is supported by an abundance of observational data, and there is little doubt that it is basically correct. The details are still being worked out, and some outstanding questions remain open. One of the big unknowns is the nature of the dark matter that manifests itself by its gravitational pull in galaxies and clusters. There are strong reasons to believe that most of this dark matter is not made up of nucleons and electrons, but rather consists of some yet undiscovered particles. The details of the galaxy formation process depend on the masses and interactions of these particles, but the general picture outlined in
Figure 4.3
does not.
Figure 4.3
illustrates the story of genesis as we have discussed it so far. This story is supported by an abundance of observational data, and there is little doubt that it is basically correct. The details are still being worked out, and some outstanding questions remain open. One of the big unknowns is the nature of the dark matter that manifests itself by its gravitational pull in galaxies and clusters. There are strong reasons to believe that most of this dark matter is not made up of nucleons and electrons, but rather consists of some yet undiscovered particles. The details of the galaxy formation process depend on the masses and interactions of these particles, but the general picture outlined in
Figure 4.3
does not.
It is truly remarkable that we can observe the universe as it was 14 billion years ago and accurately describe the events that took place a fraction of a second after the big bang. This brings us tantalizingly close to the moment of creation. But what actually happened at that moment remains as enigmatic as ever. In fact, on closer examination the big bang turns out to be even more peculiar than it seemed before.
Figure 4.3
. Abridged history of the universe.
. Abridged history of the universe.
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