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Skywatching

How big is the universe?

The universe has been described as a tremendous amount of space, which we are still trying to understand, contaminated with traces of gas and dust, in the form of:

  • cosmic clouds
  • stars
  • planets
  • other things.

How big is it? The reason we have been able to make any progress with that question is that the universe is expanding.

In the 20th Century, astronomers observing distant galaxies discovered two interesting things.

Frist, those galaxies are moving away from us.

Second, the speed at which they are receding increases with their distance from us. This is what we would see in a universe that is expanding, carrying us and all the galaxies with it.

Measurements of distances and speeds led us to one conclusion; just under 14 billion years ago everything in the universe, space and matter, were all concentrated in one small, very dense lump.

For some unknown reason, that lump then started to expand, in an event now known as the Big Bang. The universe got less dense and cooler, although still incredibly hot.

If we were there at that time we would have found ourselves surrounded by a thick, glowing fog.

Then, about 380,000 years after the Big Bang, the universe had cooled enough for stable atoms to form and the fog cleared.

The remnant glow from that fog, called today the cosmic microwave background radiation (CMB), can still be seen and mapped, because of one other feature of astronomy.

Cosmic distances are so great that as we look further and further away into space, we are looking further and further back in time. Even the light from our nearest astronomical neighbour, the moon, takes 1.25 seconds to get to us, so we see it as it was 1.25 seconds ago.

We see the sun as it was 8.6 minutes ago.

The light from the nearest star after the sun takes 4.3 years to get here.

Expressing such distances in miles or kilometres leads to appallingly large numbers, so astronomers defined a more useful measurement of astronomical distance, the "light year," which is the distance light travels in a year, just under 10,000,000,000,000 kilometres.

That nearest star lies at a distance of 4.3 light years.

Our telescopes can detect galaxies billions of light years away, taking us back to the far history of the universe. When we look at the cosmic microwave background radiation, we are looking back in time over
13 billion years, so the distance between us and the source of the CMB is over 13 billion light years.

This would suggest the radius of the universe would also be something like 13 billion light years. However, that was 13 billion years ago. Since then the universe has continued to expand.

Today, its radius should be roughly 50 billion light years. Light from the most distant parts of today's universe won't get here for a very long time.

If the universe has a definite size, does it have an edge?

Imagine ants on the surface of a balloon that is gradually being inflated.  First, all ants would see the same thing; each of them would see the other ants getting further and further away, and the more distant the other ant, the faster it is receding.

The surface area of that balloon is finite — it has a definite value, which is easy to calculate.

However, in their perambulations around the balloon, the ants would never reach an edge. The surface of the balloon is a two-dimensional universe curved into a sphere in a third dimension.

We now believe our universe is probably a similar case.

It is a three-dimensional universe being curved through a fourth dimension into a "hypersphere.”

This means we can set off in any of the three dimensions we know, and keep going forever.

If this is the case, the universe has a finite size, but no edge.

  • Mars is now lost in sunset glow.
  • Jupiter, shining like a searchlight, rises around 10 p.m.
  • Saturn can be seen around midnight
  • The moon will reach First Quarter on June 9.


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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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