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Science Bulletins: The Expanding Universe


The Expanding Universe When we talk about the expansion of the Universe,
it’s not that the galaxies and clusters of galaxies are like bullets zooming through
some preexisting space. Rather space itself, the space between galaxies
and clusters of galaxies, is expanding. In the past 10 or 15 years, we’ve discovered
that that expansion is currently speeding up with time, accelerating. Trying to figure out what’s causing the
acceleration of the Universe is perhaps the biggest unsolved problem in all of physics. Our perception of the evolution of the Universe
has certainly changed a lot with time. For example, Albert Einstein thought the Universe
is static, neither expanding nor collapsing, and so did many scientists of his time. But Edwin Hubble discovered in 1929 the Universe
is actually expanding. The Universe began its existence in a cataclysmic
event we call the Big Bang, in a sense like a gigantic explosion. The expansion of the Universe was the expansion
of all of space. That is all there was, and it was expanding. In 1998, there were two teams of astronomers
who were studying supernovae, exploding stars, at large distances from us. And they saw that these exploding stars appeared
to be somewhat fainter than we expected them to be. They realized that the simplest explanation
was that the expansion of the Universe has been speeding up over the last five billion
years or so. And this came as a real shock, because in
our conventional understanding of gravity and the behavior of matter, we would have
expected that over time the Universe would be slowing down, because all the matter in
the Universe is attracting all the other matter. Either there’s some new stuff out in the
Universe, that we call dark energy, which has the property that it’s gravitationally
repulsive so it would make the Universe speed up, or we have to admit that our understanding
of gravity has some holes in it. In order to determine what causes the Universe
to accelerate in its expansion, we need to measure more carefully the detailed expansion
history of the Universe. This can be done by looking at the expansion
as a function of time. To plot the history of the expansion rate,
we need to look at the distances of galaxies versus the speed with which they are going
away from us, or the red shift, as astronomers call it. Getting distances of galaxies is kind of like
estimating the distance of a car at night. You look at how bright its headlights appear
to be. You compare that with the known brightness
of headlights of a car of known distance, a nearby car, and in this way you get the
distance of the distant car, just by comparing the brightness of the headlights. So, in the case of astronomy, if we find stars
and galaxies whose true power, whose brightness we really know, and we measure the apparent
brightness, well, then we can determine the distance. The type of star that we use is called the
type 1a supernova. That looks good. Yeah. This to me looks like a pretty good supernova
candidate. We use KAIT, the Katzman Automatic Imaging
Telescope, to discover new supernovae. And we use the three-meter Shane reflecting
telescope to obtain spectra of those supernovae and study them in detail. This allows us to probe the extremes. I mean that’s what we need to do. That’s why I’m intrigued by this one. Right now, we know that the Universe is expanding
faster than it was four or five billion years ago, and we know that four or five billion
years ago it was expanding more slowly than another four or five billion years back before
that. So that’s sort of three data points. We’d like more measurements of the Universe. So we want to install S2 into position 30S,
which is right in the top center. O.K. The dark energy survey is a project aiming
at trying to understand what’s causing the expansion of the Universe to speed up. O.K., I have it now on this side. O.K., I’ll take it. We’re building a large camera with about
five hundred megapixels. And this is going to go on a four-meter telescope
in Chile that will study about 300 million galaxies over an area covering about 10 percent
of the sky. The reason we want to do this is to look at
the distribution of galaxies in the Universe, the large-scale structure. Galaxies are not just distributed smoothly
through space. They tend to be clumped together by gravity. Initially, the Universe was fairly homogenous,
had small lumps in it. But then over cosmic time, those lumps were
amplified by gravity. We think that that’s how large-scale structure
formed. Even though it’s been well tested on scales
of our Solar System, we haven’t been able to directly test gravity on cosmic scales. And it’s possible that gravity becomes weaker
when we get to very large distances in the Universe. So if we can measure the rate at which galaxies
and clusters of galaxies have formed, that tells us about this competition between expansion,
which is making things pull apart, and gravity, which is making them clump together. That can help us try to disentangle what’s
really causing the Universe to speed up. Is it gravity? Is it dark energy? It’s actually quite remarkable to me that
in one short decade, the reaction to the announcement of the accelerating Universe went from utter
disbelief to nearly uniform acceptance. That doesn’t mean that we understand what
dark energy is or how gravity might be wrong. And I don’t know that it’ll be resolved
in my lifetime. My hope is that as we get better data, as
we narrow down the theoretical possibilities, someone brilliant will come along and say,
“Aha! Here’s clearly what it’s gotta be.” Whether we get to that in the next 5 years,
or the next 50 or 100 years, I don’t know. But we have to keep going.

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