How Did We Get Here?

Astronomers cannot see all the way back in time to the origin of the universe, but by drawing on lots of clues and theory, they can imagine how everything began. 

Their model starts with the entire universe as a very hot dot, much smaller than the diameter of an atom. The dot began to expand faster than the speed of light, an expansion called the Big Bang. Cosmologists are still arguing about the exact mechanism that may have set this event in motion. From there on out, however, they are in remarkable agreement about what happened. As the baby universe expanded, it cooled the various forms of matter and antimatter it contained, such as quarks and leptons, along with their antimatter twins, antiquarks and antileptons. These particles promptly smashed into and annihilated one another, leaving behind a small residue of matter and a lot of energy. The universe continued to cool down until the few quarks that survived could latch together into protons and neutrons, which in turn formed the nuclei of hydrogen, helium, deuterium, and lithium. For 300,000 years, this soup stayed too hot for electrons to bind to the nuclei and form complete atoms. But once temperatures dropped enough, the same hydrogen, helium, deuterium, and lithium atoms that are around today formed, ready to start a long journey into becoming dust, planets, stars, galaxies, and lawyers. 

Gravity—the weakest of the forces but the only one that acts cumulatively across long distances—gradually took control, gathering gas and dust into massive globs that collapsed in on themselves until fusion reactions were ignited and the first stars were born. At much larger scales, gravity pulled together huge regions of denser-than-average gas. These evolved into clusters of galaxies, each one brimming with billions of stars. 

Over the eons fusion reactions inside stars transformed hydrogen and helium into other atomic nuclei, including carbon, the basis for all life on Earth. 

The most massive stars sometimes exploded in energetic supernovas that produced even heavier elements, up to and including iron. Where the heaviest elements, such as uranium and lead, came from still remains something of a mystery. 

Read More...

Flawed Gravity, or Relaxing the Grip

For almost a century, Albert Einstein’s Theory of General Relativity has reigned as the explanation for how gravity works. Einstein’s equations showed that gravity is a property of matter, and that matter "warps" the space-time around it.

Yet there are both problems with General Relativity and questions about its effects.

"Perhaps we see the acceleration of the universe and we attribute that to energy, but maybe our theory of gravity is still incomplete, so we don’t need extra energy, we just need to modify our theory."
Eiichiro Komatsu,
University of Texas at Austin

Relativity applies to the universe on large scales, but not on the smallest scales, where quantum gravity takes over. Relativity also breaks down in the presence of the strongest gravitational fields, like those at the center of a black hole.

All the experiments to date have confirmed General Relativity’s effects on large scales, but scientists still aren’t certain whether gravity has remained constant since the Big Bang, whether it acts the same in all regions of the universe, and whether it retains its grip on the very largest scales.

It’s possible that gravity weakens on the largest scales – across billions of light-years – and thereby allowing the universe to expand at a faster rate as it grows larger.

Read More...

New Physics, or Particles and Fields

Scientists have discovered a menagerie of particles, and suspect that the universe contains many more — perhaps including particles of dark energy and dark matter. 

Physicists have observed lots of particles in the universe: the protons, neutrons, and electrons that make up atoms; the quarks that make up protons and neutrons; and particles with such exotic names as muons, leptons, and neutrinos. At the same time, they have observed many "fields" that play important roles in the universe: magnetic fields, electric fields, and gravitational fields, among others.

And they are looking for even more. The world’s largest particle accelerator, which is scheduled to begin operations in Europe in 2008, will search for a particle called the Higgs boson, which may be responsible for the "field" that gives mass to some particles but not others. In fact, particles and fields are just different descriptions of the same phenomena. Just as matter and energy are equivalent, as Albert Einstein described in his theories of relativity, particles and fields are equivalent, too.

The search for new particles and fields extends to the search for dark energy and dark matter.

Dark matter fills the universe, and exerts a gravitational pull on the "normal" matter, like stars and planets, around it. Yet it produces no detectable form of energy – no light, heat, radio waves, or anything else. Some physicists believe that dark matter is a form of particle that was produced in the Big Bang but that has not yet been detected.

Dark energy may consist of an undiscovered energy field that permeates the universe, and that may change over time.

Dark energy, of course, is still in the early stages of study. Physicists who feel that vacuum energy has too many problems are looking for other solutions, and one contender is a new type of particle. Dark-energy particles likely would have been created in the Big Bang as well, and would permeate the entire universe.

Others describe a new field that permeates the universe, called quintessence. One key difference from the vacuum energy is that quintessence would vary with time, so it might not show up at all in the early universe, but exert a powerful influence today. There are several hypotheses of quintessence fields, and in each one, each with a different outcome for the future of the universe.

So far, there’s no evidence of dark energy particles or fields. Discovering them would require not only the efforts of astronomers, but new experiments for even more powerful particle accelerators, which are decades away.

Read More...

Vacuum Energy, or Einstein’s Blunder

1. Empty space. 2. Two particles suddenly appear. 3. Particles ram together and annihilate each other. 4. They leave ripples of energy through space.

If Las Vegas were taking bets on dark energy, the odds would favor a concept known as vacuum energy or the cosmological constant. In essence, it suggests that space itself produces energy, which is "pushing" the universe outward.

Albert Einstein invented the cosmological constant as part of his theory of gravity, known as General Relativity.


1. Empty space. 2. Two particles suddenly appear. 3. Particles ram together and annihilate each other. 4. They leave ripples of energy through space. [Tim Jones]
Einstein’s equations showed that the gravity of all the matter in the universe would exert a strong pull, pulling all the stars and galaxies toward each other and eventually causing the universe to collapse. At the time, though, astronomers believed that the universe was static – that it was neither expanding nor contracting. To counteract this problem, Einstein added another term to his equations, called the cosmological constant, to balance the inward pull of gravity.

Within about a decade, though, astronomer Edwin Hubble discovered that the universe is expanding. Einstein discarded the cosmological constant, calling it his greatest scientific blunder.

When dark energy was discovered, though, many physicists began to think that Einstein’s only blunder was in removing the constant. This "repulsive" force could begin to explain the acceleration of the universe. In other words, it might be the dark energy.

Today, physicists explain the cosmological constant as the vacuum energy of space.

In essence, this says that pairs of particles are constantly popping into existence throughout the universe. These "virtual pairs" consist of one particle with a negative charge and one with a positive charge. They exist for only a tiny fraction of a second before they collide and annihilate each other in a tiny burst of energy. This energy may be pushing outward on space itself, causing the universe to accelerate faster.

One of the appealing elements of vacuum energy is that it could explain why the acceleration has only started fairly recently on the cosmic timescale.

In the early universe, all the matter was packed much more densely today. In other words, there was less space between galaxies. With everything so close together, gravity was the dominant force, slowing down the acceleration of the universe that was imparted in the Big Bang. In addition, since there was less space in the universe, and the vacuum energy comes from space itself, it played a much smaller role in the early universe.

Today – 13.7 billion years after the Big Bang – the universe has grown much larger, so the galaxies are not packed so close together. Their gravitational pull on each other is weakened, allowing the vacuum energy to play a more dominant role.

Vacuum energy has its own set of problems, though. It should be far too weak to account for the acceleration seen in the present-day universe, for example — by a factor of at least 1057 (a one followed by 57 zeroes), and perhaps as much as 10120 (a one followed by 120 zeroes). Yet it is the most complete scenario to date, so it leads the pack of dark-energy contenders.

Read More...

What is dark energy?

Like the "dark side of the Moon," dark energy represents the unknown. In the late 1990s, astronomers discovered that the universe is expanding faster today than they had expected. But they don’t know what is causing the acceleration, so for now, they simply call it "dark energy."

"Dark energy is our ignorance of what’s going on in the universe right now," says Karl Gebhardt, a professor of astronomy at The University of Texas at Austin and one of the principal investigators for the Hetdex project

"What I always like to say is that dark energy is only a phrase, and don’t get hung up on the words dark and energy. [Dark energy] may not be dark, and it may not be energy. All it is is our ignorance of how the universe may be expanding, and we don’t know what it is at this point."

Even so, theorists have already developed several explanations for dark energy. These explanations include an energy born from space itself, new kinds of subatomic particles, and even a flaw in the theory of gravity. Hetdex and later experiments will allow physicists to select the correct one.

"Whatever the answer is," says Gebhardt, "it’s going to be a fundamental change in our understanding of the basic properties of the universe."

Vacuum Energy, or Einstein’s Blunder

If Las Vegas were taking bets on dark energy, the odds would favor a concept known as vacuum energy or the cosmological constant. In essence, it suggests that space itself produces energy, which is "pushing" the universe outward.

New Physics, or Particle X

Physicists have observed lots of particles in the universe: the protons, neutrons, and electrons that make up atoms; the quarks that make up protons and neutrons; and particles with such exotic names as muons, leptons, and neutrinos.

Flawed Gravity, or Relaxing the Grip

For almost a century, Albert Einstein’s Theory of General Relativity has reigned as the explanation for how gravity works. Einstein’s equations showed that gravity is a property of matter, and that matter "warps" the space-time around it.

Read More...