How old is the universe? Notre Dame physicist Grant Mathews has one birth date in mind -- 15 billion years ago -- and says he can explain why that's almost twice as old as one popular estimate.
In calculating the age of the universe, much depends on where you look for evidence. Mathews' reference points of choice are the million-star globular clusters that orbit our Milky Way galaxy above and below the stellar disk. The stars in these globular clusters were among the first to form after the big bang, he says. If, as their condition suggests, the clusters are 15 billion years old, then the big bang probably went off a galactic eye blink earlier, say, a few million years before.
Other scientists have other ideas. Some focus on the ages of the oldest, dimmest of the white dwarfs, which are what stars like the sun become after exhausting their hydrogen and helium fuel. The oldest white dwarfs discovered so far appear to be about nine billion years old, making the universe at least that old.
White dwarfs are faint, so only nearby ones made after the Milky-Way disk began to form, several billion years after the big bang, can be detected. Older white dwarfs are probably out there, Mathews says, but they are too far way to detect.
Another way of figuring the age of the universe uses the decay rate of such radioactive elements as uranium and thorium, which are believed to have been formed in ancient massive stars. Calculating backward using those elements' decay rates suggests that the element-making star furnaces began working some 12 billion years ago, assuming stars and elements were formed continuously. However, if there was a gap between the initial first star formation and the star formation in the Milky Way, then the radiologically determined age of the universe could be much older, Mathews says.
More recently, some scientists are using images from the Hubble space telescope to date the universe. By calculating the rate at which nearby galaxies appear to be moving apart and by running these rates in reverse, these researchers suggest that the universe may have been all in one piece as recently as eight billion years ago.
That estimate, however, assumes the universe has been expanding at a continuously slowing rate. It's possible that the expansion rate has been coasting at a constant rate for some time, Mathews says. This would happen if the expansion energy exceeds the gravitational pull from all of the galaxies.
It is even possible, he says, that the expansion rate has been speeded up recently by so-called vacuum energy, a force first postulated by Einstein that could be generated by infinitesimally small particles interacting in the vacuum of space. If the universe is expanding faster now than in the past, Mathews says, one would underestimate the age of the universe by merely working backward from the present expansion rate.
"My favorite theory is that this vacuum energy is small but finite and we are living in the epoch when it is just starting to become important," the Notre Dame physics professor says.
The question foremost in many physicists' minds is whether the universe will continue to expand forever or if enough mass and gravity exist to force everything back into a pre-big-bang configuration. Mathews thinks the universe could expand forever because of the push from vacuum energy.