Kate Becker
Kate Becker

We are not special.

I don't mean you and me personally, or even humanity as a whole. For all we know, human beings are authentically special creatures in the cosmos. I'm talking about our sun, our solar system, our galaxy -- our entire cosmic neighborhood.

Today this is a fundamental tenet of astronomy, but it hasn't always been this way. Until astronomers and philosophers came around to the idea that the sun, and not the Earth, was at the center of the solar system, our planet occupied a unique and privileged place in the cosmos.

You might say that the last 500 years of astronomy have been about taking us down a notch -- and then another notch, and then another -- as we've realized that not only is our planet not the center of the universe, but neither is our star, nor even our galaxy.

Yes, it is a cosmic bummer, but it's also a very useful tool for cosmologists, who get business done by assuming that most places in the cosmos operate pretty much like our home turf. When we assume that the laws of physics are the same from the Milky Way to the Andromeda Galaxy to the most distant quasars, we can apply our hometown physics to far-flung objects and actually draw scientific conclusions about them.

If every corner of the cosmos followed its own house rules, after all, we would never know how to play the game outside our own little corner of town. As astronomer William Keel put it, we must assume that the universe is "playing fair" with us.

In fact, astronomers have amassed plenty of evidence to back up this claim. On the largest scales, galaxies cluster along the strands of a vast, illuminated web that looks essentially the same in every direction. We also see that galaxies on every side of us are rushing away at about the same speed.

Finally, observations of the cosmic microwave background radiation, the echo of the very early universe, reveal that our universe was "baked" from a consummately smooth batter -- one so smooth that it varies only one part in 100,000.

This principle of "unspecialness" can be extended to time, too. Not only is our corner of the cosmos utterly ordinary, so is the era in which we happen to be living. Of course, there have been special times in the history of the universe -- the Big Bang and the years shortly thereafter -- but the last few billion years and the billions to come will look more or less the same.

Yet perhaps our place in time isn't so ordinary after all. Avi Loeb, a theorist at the Harvard-Smithsonian Center for Astrophysics, has analyzed what the cosmos would look like to astronomers throughout the history of the universe.

If our scientists could somehow time travel back more than 13 billion years to a moment just 500 million years after the Big Bang, they would get the best possible view of the "lumps" in the batter of the cosmos, seeing them just as they began to evolve into the stars and galaxies that cover our sky today. Yet these time-traveling astronomers would also have the advantage of coming a bit late to the party; the 500 million years since the Big Bang would be time enough for light from distant parts of the universe to reach them.

Our view today may not be optimal, says Loeb, but at least we have a view. Our universe is expanding faster every moment, propelling distant galaxies out of reach of our telescopes. Once they are beyond the horizon, they are forever inaccessible. Some time between 100 and 1,000 billion years from now, Loeb calculates, we will only be able to see objects to which we are bound by gravity.

At that moment, the universe will look very much like it did a millennium ago: We will be special once again. But we will also be alone.
