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Using ground-based telescopes or the Hubble Space Telescope, astronomers take photographs of galaxies which are many light years from the Earth. Does it mean those photographs are as old as the galaxies are far from the Earth?

As I read that the Universe is growing very radidly, then its position and shape should have changed after many years. Are photographs useless as a result?

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3 Answers 3

up vote 6 down vote accepted

In addition to Rory's answer, studying "old" data is arguably more valuable than if we had access to a single snapshot of the entire visible Universe. By looking out, we are also looking back in time. That means scientists can see how the Universe has changed -- they can compare the structure and behavior of stars and galaxies as they were "just" 100M years old with the structure and behavior as they were 10B years ago.

For instance, we can tell that space was not always expanding at the rate it is now. That's an enormous discovery that we would not have been able to make with a "snapshot" view of the Universe.

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When we photograph a galaxy very far away, the light has traveled a long time to reach us, so the image we see is indeed the galaxy as it looked at the time the light was emitted. However, this does by no means make the image useless - on the contrary!

Knowing that looking far out in space also means looking back in time, we can (if our telescopes and scientific models are good enough) see all the way back in time to the Big Bang - or at least a short while after the Big Bang, when the Cosmic Microwave Background was emitted (we can not see further out and back than that, because the Universe was not transparent at that time).

This means that basically, the entire evolutionary history of the Universe is out there for us to look at, if we can just figure out how to see it. Not many years ago, we could only see galaxies out to a redshift of about 1 (about 8 billion years ago, when the Universe was about 6 billion years old), but today, the furthest galaxies we have seen have a redshift as high as 8, corresponding to looking 13 billion years back in time, to when the Universe was only about 650 million years old. The Cosmic Microwave Background has a redshift of about 1100 (that is, the wavelength is stretched to around 1101 times their original value), meaning that it has been emitted when the Universe was only around 300,000 years old.

It's not really a question of whether the images are "useless" or not, but rather what they are useful for. Of course, they are rather useless to figure out what the galaxy looks like today. But they are very very useful for finding out how galaxies evolve, and how they have formed the large scale structure that we observe today, as it can be seen in the famous Millennium Simulation.

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Those photographs are very old - literally, if something is 1 million light years away from us, any photograph we take is 1 million years old. It is that simple.

The photographs are definitely not useless, as we can't travel faster than light - and neither does anything else, so any calculations or assumptions we make based on what we see in these photographs is effectively real for us, as wherever that galaxy is now (a million years later than the photograph) is irrelevant to us - there is no physical way for us to interact with it.

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