In more than twenty years of hard work, the Hubble Space Telescope (HST) has made both beautiful images and profound discoveries, sometimes doing both at the same time. What are its greatest contributions to science? Hubble has enabled us to estimate just how big and how old the universe is and in the process found the most far away things we have ever seen.
Here’s something you can do yourself: go to a good library and look at their astronomy books from before the HST was put in orbit (say from 1975 to 1990) and see how old they say the universe is. I’ve just done that and found quoted ages between 10 and 20 billion years since the Big Bang, the best estimate obtainable with the finest instruments available at the time. Never mind the numbers, think about the degree of uncertainty; if you looked at another person to estimate their age and could only say that they were aged between ten and twenty, it might be time to sharpen your vision by investing in a new pair of glasses. In a way that is what the astronomical community did when the Hubble Space Telescope became fully operational (after the STS-61 servicing mission in 1993); astronomers got a new and sharper vision of deep space.
To measure the size of the universe, you would look for particular types of star which have a known brightness. Once you find one of them, you measure how bright it appears to you after its light has crossed bazillions of kilometres and compare this to how bright you know it actually is. Using very straightforward maths you can then calculate how far away it actually is. One especially useful type of star for this kind of measurement is known as a Cepheid variable. Cepheids pulse at a rate directly related to their brightness, so you just look out for the pulsation rate to find their intrinsic brightness. This technique was mature by the 1920s, and Edwin Hubble himself used it to measure the distance to the Andromeda galaxy by observing the Cepheids there.
One of the Hubble Space Telescope’s goals was to accurately observe Cepheids in the most distant galaxies known (we knew they were far away, the problem was saying just how far). Astronomers could then precisely measure just how distant they are. This work is becoming ever more refined; in January this year astronomers revealed that they had identified the farthest galaxy yet seen in a Hubble image.
In this Hubble Ultra Deep Field (UDF) image made with the HST there is a faint fuzzy splodge unromantically identified as UDFj-39546284 . This is a whole galaxy of a billion stars (only 1% of the size of Milky Way) about 13.2 billion light years away making it the most remote thing we have ever seen, beating the previous record-holder by 150 million light years. (Of course, what is the most far away galaxy is a matter of perspective: any intelligent beings living in UDFj-39546284 could look at the Milky Way and see it as the most distant object in the Universe!)
Observing UDFj-39546284 is fascinating to astronomers we are seeing it as it was during an ancient period called the “Age of Reionisation”. This marks the end of the universe’s earliest phase when all of space was filled with an opaque murk of hot hydrogen gas which blocked starlight (so some people call this the “Dark Ages” of the universe). The age of reionisation ended when this gaseous soup thinned out enough to let starlight shine freely through space. The light from the stars of UDFj-39546284 began its journey just 480 million years after the Big Bang, making it some of the oldest light possible to be seen!
Accurately measuring distances to far away objects has a side effect: it leads to a better estimate of the age of the universe too. Edwin Hubble famously discovered that the universe is expanding and the more distant the object the greater its speed as it is carried along by the space expanding around it. This is known as Hubble’s law. Now, if you know how fast the universe is getting bigger, you can work backwards, shrinking the cosmos in your imagination, squeezing the galaxies closer and closer until they are all mushed together at a single point. Effectively, you have wound time backwards to the Big Bang, and knowing how long this takes gives you the age of the universe since the Big Bang (sometimes called the Hubble Time).
Thanks to the Hubble Space Telescope’s superb observations we can say that the universe is 13.75 billion years old. Compare the best we could do before the HST, between ten and twenty billion years with 13.75 billion (it could be 170 million years older or younger, even the HST isn’t perfect) and you can see how important this fantastic instrument has been to our vision of the universe we live in.
Forget the TARDIS, the Hubble Space Telescope is the real space and time machine!