» Invisible universe: interview with George Smoot
03.07.2009 -

Invisible universe: interview with George Smoot

Transcript of Video: George Smoot, Nobel laureate in Physics, talks about dark matter and dark energy.

Question: What are dark matter and dark energy and what do we mean by the “invisible universe”?

In terms of total energy content 95% of the universe is not conventionally visible, so we give it the name dark matter and dark energy. In the early days, people thought it was ordinary matter that was just dark and so you could see it with light. However, later we came to realize it is probably a kind of matter, unlike the matter in this room, which does not interact strongly with light. Instead, it only interacts gravitationally with light.

Based upon this characteristic, a better name for dark matter would perhaps be “invisible matter”. What this means is that we don’t see it directly as one would see a room or another person. Instead, we see it indirectly from its influence on gravity and from the impact it has. In this way, it is like the invisible man where you can see the footprints. In this case, however, it is the invisible matter or the invisible energy and you see the effect on the light particles, or the ordinary particles, being moved around by its gravity.

How much dark matter is there in the universe?

From observations we know that roughly 4.5% of all the stuff in the universe by weight or by energy is the kind of matter we can see around the room. About 25% of the universe, however, is matter altogether. This means there is at about a 5 to 1 ratio of ordinary matter to this dark matter, i.e. the substance that provides the framework or background in which the visible matter, the stars and the galaxies, are located. So that’s the issue with the dark matter.

What does dark energy tell us about the future of the universe?

Dark energy was discovered 10 years ago when trying to trace out the curve of the rate of change of separation between galaxies as a function of time for the universe. As you look out further and further into the universe, in other words, you see further and further back in time. What you saw and what people expected is similar to the curve of a ball when you throw it into the air. The trajectory of the ball would produce a curve that eventually would reach its peak and then turn and go down. The question was, “Will the universe curve like this and eventually reach its peak and then collapse?” or “Will it keep going up forever, but slower and slower?” What we found 10 years ago was that it was going up, but instead of getting slower and slower, it was way faster. It is like you threw a ball into the air and it is accelerating. It was as if it was slowing down and slowing down and slowing down and then suddenly it accelerated and went away.

How do we explain this?

One of the ways you can explain this phenomenon is by changing the law of gravity. Another way is to say that there is a new substance in the universe, which unlike matter, doesn’t clump under gravity. Instead, it spreads itself out uniformly. As a result it causes the space to expand more and more rapidly; to accelerate the rate of expansion. Therefore, dark energy is really a stalking horse for a possible new constituent of the universe or for the fact that the universe is accelerating its rate of expansion. It may be modified gravity or it may be a new substance in the universe. Most of us guess it is a new substance in the universe.

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