The importance of the Big Bang cannot be overstated: it is thought to be the “moment” at which space and time themselves began. Physicists can say nothing about what created the Universe, or why, but our natural curiosity leads us to try to picture how it all began.
It seems barely credible, but the definitive proof of the Universe’s moment of creation was at first dismissed as pigeon droppings. In 1964, two researchers from Bell Laboratories, New Jersey, began to pick up a strange signal through a radio telescope. At first, they thought it might be interference from New York City. When they investigated and rejected that idea, they noticed that the telescope structure was home to a pair of nesting pigeons who had peppered the device’s sensitive surfaces with droppings. So they cleaned the telescope and shipped the birds across the state. Pigeons being pigeons, however, they flew home and took up residence once more in the telescope. It was necessary to resort to a final solution: a man with a shotgun. Yet even with the pigeons dispatched and the telescope scrubbed clean again, the mysterious signal remained. The researchers, Arno Penzias and Robert Wilson, began to think that it might be coming from space.
Unknown to them, a group of astronomy theoreticians had predicted its existence almost two decades earlier. In 1948, the Ukrainian physicist George Gamow had been exploring the consequences of Georges Lemaître’s 1927 idea that the entire Universe exploded from a single compacted “atom” sometime in the remote past (see How Old is the Universe?). Gamow used this idea of a Big Bang to show that it explained the overwhelming quantity of hydrogen and helium in the Universe. He calculated that, if the Universe began with nothing but the simplest chemical element, hydrogen, then the intense heat of the Big Bang would have fused a quarter of it into helium: almost the exact proportions that astronomers were seeing in their inventory of the Universe. Gamow also made the prediction that radiation from the fireball that forged the helium would still be lingering in the Universe today as an all-pervasive blanket of microwaves. So, when Penzias and Wilson began talking about detecting microwave static from outer space, they caused a sensation. Cosmologists quickly pointed out that the pair had discovered nothing less than the residual radiation from the Big Bang.
“If you wish to make an apple pie from scratch, you must first invent the Universe.”
CARL SAGAN 20TH CENTURY ASTRONOMER
Current physics cannot describe the very beginning of the Big Bang, because it cannot deal with the tiny fractions of time and space that would need to be considered. The smallest unit of time that physics can presently handle is 10–43 seconds: a decimal point followed by 42 zeros and a one. It is known as the “Planck time,” after Max Planck, the father of quantum theory (see What is a Black Hole?). All we can say is that during this time, termed the “Planck era,” everything that we see in the Universe today was squeezed into a tiny dot, smaller than an atomic nucleus. The four fundamental forces of nature—gravity, electromagnetism and the strong and weak nuclear forces—were indistinguishable from one another, and the “dot” was already expanding. To fully picture the Planck era, a theory of quantum gravity is needed, such as string theory (see Was Einstein Right?).
As the Planck era ended, so gravity became a distinct force and physics as it is presently understood took over. However, the temperature and pressure were so extreme that matter and energy were entirely interchangeable; particles would form spontaneously from the writhing energy. Every time this happened, out too would leap particles of unusual stuff: antimatter.
THE UNIFICATION OF NATURAL FORCES: THE FOUR FUNDAMENTAL FORCES OF NATURE WERE ONCE JOINED TOGETHER
The concept of antimatter first found its way into the minds of physicists in 1928 when British physicist Paul Dirac discovered an equation that correctly described the behavior of electrons moving at high speed, but predicted that “mirror-image” electrons should exist as well. These other electrons would be identical in mass but would carry positive electrical charge instead of the normal negative charge. Just four years later came the experimental proof, with the discovery of a positively charged electron (later called a positron) in a shower of particles coming from space. Dirac extended the idea to all particles of matter, and coined the umbrella term of “antimatter” for the oppositely charged counterparts. One of antimatter’s properties is that, should it run into its mirror-image piece of matter, both will transform into pure energy. For example, the collision of a positron with an electron will annihilate both, giving out a pair of gamma rays.
“Why is there something rather than nothing?”
GOTTFRIED LEIBNIZ 17TH CENTURY PHILOSOPHER
This leads to one of the most vexing questions for modern cosmology: why is there any matter left in the Universe? Since an antimatter counterpart is predicted to accompany every particle’s creation, it should mean that everything eventually annihilates back into energy. Yet the existence of stars and planets and galaxies shows clearly that there is a residue of matter.
The solution is bizarre: mathematical calculations reveal that for every billion particles of ordinary matter created after the Big Bang, there were only 999,999,999 particles of antimatter formed. These annihilated with the corresponding matter, leaving a single orphaned particle of matter. This process repeated over and over again in the early Universe, building up enough matter particle by particle to make all the celestial objects. It means that for every particle of matter that exists today, there were once a billion other particles, but all of these were annihilated back into energy—which eventually became the microwave background radiation we see today.
In the period following the Planck era when matter was being created, cosmologists believe that the Universe underwent a sudden intense expansion that they call “inflation,” which drove the Universe to balloon by a factor of 1050 in a time of just 10–32 seconds. The observations that led astronomers into this thinking are encapsulated in two thorny cosmological problems.
“The Universe is a big place, perhaps the biggest.”
KURT VONNEGUT 20TH CENTURY SCIENCE FICTION WRITER
The first is the horizon problem. This is best explained by reference to the cosmic microwave background, which shows that the temperature on one side of the Universe is the same as on the other. The energy carried by the microwave radiation governs the temperature of space by heating any molecule or atom that gets in its way to the same temperature of about 2.7 kelvin (–270.3 degrees Celsius). The conundrum is why it should be the same everywhere, when the two far sides of the Universe cannot yet know of the other’s existence. In scientific parlance, they are said to lie outside each other’s observable horizon. Separated by at least 26 billion light years and probably much more (see How Big is the Universe?), the opposite sides of the Universe have not had time, in the Universe’s lifetime of 13.7 billion years, to exchange energy and so equalize their temperature. Yet, the entire Universe displays the same temperature, no matter where astronomers look. They do not know how this could happen, so they call it the horizon problem. The inflation theory supplies a possible answer because it states that the entire volume of our Universe came from a vanishingly small region that suddenly grew in size and spread the same temperature across space.
Second is the flatness problem. Einstein’s proposition that matter curves the fabric of space means that the Universe should have an overall curvature, determined by the total amount of matter and energy it contains (see What Will Be the Fate of the Universe?). But as far as anyone can tell, the Universe is completely “flat” on the largest scales, an extremely unlikely outcome of Einstein’s equations that represents a perfectly balanced cosmos. Again, inflation could provide a solution because whatever intrinsic curvature the Universe has, it has become spread over such a large scale that we can no longer perceive it. This is exactly like the way the ground beneath our feet appears flat even though we know it is part of the curved surface of the Earth.
While inflation helps with these problems, the question remains as to why the Universe would have inflated in this way. It may be linked to the way the strong nuclear force “broke away” from the electroweak force (the still-united weak nuclear force and electromagnetic force). This separation may have produced energy and driven the inflation, but this is uncertain and the subject of much debate between physicists.
THE FLATNESS PROBLEM: INFLATION FLATTENS THE UNIVERSE, NO MATTER WHAT ITS ORIGINAL SHAPE
By the time the Universe in cosmological models reaches a millionth of a second in age, there is more confidence about what is going on. This is because powerful particle accelerators, such as the Large Hadron Collider in Switzerland, can recreate the high-temperature, high-pressure conditions by smashing particles together with great energies and analyzing the debris.
At a microsecond, the Universe was filled with subatomic particles called quarks, together with antiquarks and gamma-ray photons. Quarks are the smallest building blocks of ordinary matter. These began to gather themselves together to form protons and neutrons, which in turn went on to form the atomic nuclei of today. As time ticked forward, the Universe continued to expand—although much less rapidly than in the inflation era—lowering the density of matter and energy, and so reducing the temperature and pressure. At around two seconds after the Big Bang, the electroweak force separated into the weak nuclear force and electromagnetism; now all four of the fundamental forces displayed their unique characteristics. This led on to the next great era of the Big Bang, which began about three minutes after creation, and ended perhaps a quarter of an hour later. It is known as the era of “nucleosynthesis” and is the period George Gamow investigated mathematically during the 1940s.
For these few minutes, the entire Universe was somewhat similar to the interior of a star; in this hot, dense maelstrom, some of the protons and neutrons combined to form helium and lithium nuclei. When formulating his theory, Gamow originally thought that all the elements could be forged in the aftermath of the Big Bang, but as he looked at the calculations he realized that they only worked well for hydrogen, helium and lithium; subsequent investigations confirmed that the Big Bang would not build anything heavier than lithium. The existence of heavier atoms in the Universe today became a mystery that was solved in 1954, when British physicist Fred Hoyle showed that the nuclear furnace at the center of a star is the only place where the rest of the chemical elements can be built (see Are We Made from Stardust?). As a result, we now know that heavy elements did not appear in the Universe until nearly a billion years after the Big Bang and this has implications for the nature of the first celestial objects that could form (see What Were the First Celestial Objects?).
“There is a coherent plan in the Universe, though I don’t know what it’s a plan for.”
FRED HOYLE 20TH CENTURY ASTRONOMER
The Universe then entered a relatively calm phase, becoming a sea of jostling particles and photons of energy. The photons, created by the matter-antimatter annihilations, continually collided with electrons, preventing them from bonding to the atomic nuclei, and thus creating a state of matter called plasma. Plasmas exist today inside stars and in clouds of gas surrounding high-mass stars; in the early Universe the plasma stretched across all space.
By the time the Universe was a year old, collisions between photons and particles had become less frequent because the continuing expansion gave the particles more space in which to move about. This allowed gravity to begin slowly pulling the particles into clumps. As time passed and the density of the Universe continued to decrease, around 380,000 years after the Big Bang one of the greatest watersheds was reached—the “decoupling” of matter and energy. All of a sudden, the drop in number of electron-photon collisions meant that electrons could be captured by the atomic nuclei. Neutral atoms of ordinary matter formed, and space was cleared of its fog of particles. Some say that this was the point at which the Universe became transparent, because most photons could now travel all the way across the Universe without running into an absorbing particle. At this stage the photons made up a powerful sleet of X-rays; they have been continually redshifted by the expanding Universe ever since and now exist as the microwave background detected by Penzias and Wilson. In the future they will become weaker still, stretching out to become mere radio waves, the weakest form of electromagnetic radiation.
Maps of the microwave background radiation in all directions have been analyzed and found to contain minuscule variations in its temperature from place to place. These “anisotropies” amount to differences of no more than a hundred-thousandth of a degree, but are highly significant. They are the imprints left on the microwaves by the clumps of matter that formed in the early Universe, around its 380,000th birthday, and show us the seeds from which galaxies and the large-scale structure of the Universe emerged.
To get a snapshot of the Universe at a younger age, astronomers must turn to tiny particles known as “neutrinos,” which appear during interactions involving the weak nuclear force. About two seconds after the Big Bang, the moment that the weak force separated from the electromagnetic force, a gigantic burst of neutrinos would have been released. This should be all around us even now, just as the microwave background fills all space. Neutrinos, however, are far more difficult to observe than microwaves. They are ghostly particles that find it easy to slip right through detectors, in fact they stream unnoticed through the entire Earth. In the time it has taken you to read this page, trillions of them will have passed through you, into the Earth and out of the other side to continue their journey across the Universe.
The possibility of such flighty particles first presented itself in calculations during the 1930s, but it took almost three decades before a neutrino was captured in a purpose-built detector. The early versions of these detectors resembled giant swimming pools, but buried deep underground so that the surrounding rocks could shield them from unwanted particles. They were filled with water or some other detecting fluid and lined with sensors that would record the flash of light produced when a neutrino struck a molecule in the tank. Typically they would capture one or two neutrinos a month. Modern neutrino detectors can be found buried in the Antarctic ice, or sunk below the surface of the Mediterranean Sea. They still look for flashes of light produced by neutrino collisions, but they scan the ice itself, or seawater, for the faint flash that betrays each neutrino’s passage. It is hoped that within a decade these detectors will provide a neutrino map of the entire sky. Unfortunately for cosmologists, however, it is not the low-energy neutrinos given out two seconds after the Big Bang that will be mapped, but high-energy ones generated from the explosion of stars. Nevertheless, this will be a step in the direction toward a grabbing a picture of the Universe as it looked when it was two seconds old.
Cosmologists’ ambitions do not stop there. The ultimate picture of the Big Bang may be possible if physicists can unify the forces and deduce the nature of the particles suspected to carry quantum gravity, the gravitons. By analogy with the weak nuclear force and neutrinos, the separation of gravity at the end of the Planck era would have generated a background of gravitons. There is speculation regarding a future graviton telescope, which would be able to detect these and so give a picture of the Universe just 10–43 seconds after the Big Bang. To all intents and purposes, it would be a picture of the Big Bang.