THE EXPANDING UNIVERSE
is to decide that the law of gravity needs altering, but this is unpalatable to most astronomers.)
Dark energy is causing the universe to expand at an ever-faster rate. It represents 68 percent of the universe. The discovery of dark energy in the 1990s was a surprise, because the expectation in cosmology had been that the gravity of all the matter in the universe would slow down the expansion discovered by Edwin Hubble. Think of the universe as having a brake (gravity) and an accelerator (dark energy), with both being pushed at the same time. Currently, the accelerator is twice as strong as the brake, so the universe is accelerating.
We know far less about dark energy than dark matter, but it seems to be a property of space. Physics tells us that space is not nothing — it has the potential to create energy. Albert Einstein formulated a version of his gravity theory where the energy in empty space is not diluted as space expands. As more space comes into existence, more space energy appears, causing the universe to expand faster and faster. So, the idea that the amount of dark energy grows as the universe expands has been around for a while. But we still lack a physical explanation to test this idea.
Are dark matter and dark energy stable and constant? Since we don’t understand their true physical nature, we can’t be sure. But astronomers can see if they vary depending on which direction in space they look. This is a test of whether the universe is lopsided or the same everywhere (the physics term for this is isotropic). It turns out that the amount of dark matter surrounding galaxies is the same in every direction, and the strength of dark energy is also the same in every direction.
To see whether the influence of dark matter and dark energy has changed over cosmic time, astronomers look deep into space. Distant light is old light, so telescopes act as time machines, probing billions of years into the past. By measuring the redshift and brightness of distant objects, astronomers map out the expansion history of the universe. Dark matter dominated for most of that history since the Big Bang. That’s because when the universe was smaller, the gravity exerted by dark matter was stronger, while the force exerted by dark energy has stayed the same. Now is the only time in the entire history of the universe when the two entities’ influences are about equal. In the future, the effects of dark energy will increasingly dominate, and the universe will accelerate forever.
Chris Impey Distinguished Professor, Department of Astronomy, University of Arizona, Tucson
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HOW DO GLOBULAR CLUSTERS REMAIN INTACT FOR SO LONG? AS STARS ORBIT THE COMMON CENTER OF MASS, SHOULDN’T THEY CROSS ORBITS AND COLLIDE REGULARLY, DESTROYING THE CLUSTER IN RELATIVELY SHORT ORDER?
Terrence Schell
A IGlobular clusters are ancient, spherical groups of stars that are often as old as the galaxies they orbit. The stars in a globular cluster orbit the center of mass of the cluster, and the angular momentum of the stars as they move in their orbits keeps the cluster from simply collapsing in on itself. This is the same reason the planets of our solar system don’t fall into the Sun.
But what about stars within the cluster colliding? There are a few factors at play here. First, remember that the stars are always moving — to get a star-star collision, you would have to have two stars whose orbits cross both meet in the same place at the same time. This is like trying to hit one moving target with a second moving target. It’s not impossible, but it is unlikely.
And second, although stars in a cluster are closer together than out in the field (i.e., not in a cluster), the average distance between two stars in a globular cluster is still about 1 light-year. That’s quite far apart! So, most orbits aren’t likely to cross.
Of course, there are exceptions: Stars are only an average of 1 light-year apart, so some are much closer, down to a few light-hours apart — the size of our solar system — or less. So, despite all the reasons I’ve just given for why collisions are not the overall norm, stars can and do collide, particularly in the centers of the most densely populated globular clusters. Astronomers think such collisions might be how certain stars called blue stragglers are created. These stars are particularly massive and bright, meaning they should not live long, yet they are found in these ancient clusters. One way such a star could be produced is if two smaller, older stars collide, creating one massive star that suddenly has a lot of new fuel to burn and looks artificially young.
Even though most stars in a globular cluster are unlikely to collide, stars do often interact with each other gravitationally. If two stars pass close enough to each other, they might exchange energy, giving one a boost so it moves faster and perhaps even orbits a little farther out than before, while the other loses energy and orbits a little slower and closer to the center. In this way, globular clusters change dynamically over time, with heavier stars sinking toward the center and lighter stars moving to the outskirts or perhaps getting kicked out of the cluster altogether.
Alison Klesman Senior Editor