Our galaxy, the Milky Way, is just one of an enormous number of galaxies spread throughout a vast Universe, which is expanding.Everyone knows these facts now, but how do we actually know about our place in the Universe?
Light travels through space as a wave, and hence light has a wavelength.If you imagine waves on the ocean, their wavelength is the distance between each crest.Just for a moment think about the familiar colours of the rainbow (or spectrum).At one side we start with red which has a long wavelength, next orange with slightly shorter wavelength, moving through the colours to blue and violet the wavelengths get smaller and smaller.
Travelling as a wave has an important consequence for light.Light waves from moving sources are stretched or compressed by the motion.If an object is coming towards you its light is blueshifted meaning the light waves are squeezed together, making them appear more bluish, similarly if it is receding the light is redshifted.The speeds would have to enormous for this effect to be noticeable to the eye, so approaching car headlights will never appear blue!This phenomena is called the Doppler Effect and is very useful in many aspects of astronomy.For example, by studying the light from a star we can tell if it is coming in our direction (blueshift) or moving away (redshift), or we can use the Doppler Effect to measure its speed.
Between 1912 and 1917, astronomer Vesto Slipher (1875-1969) was using the Doppler Effect to measure the speed of ‘nebulae’, at the time still believed to be nearby objects inside our own galaxy.To his surprise, of twenty five nebulae, four were blueshifted (and therefore coming towards us) and the rest were redshifted and hence moving away from us.Edwin Hubble (1889-1953) had heard of this and was fascinated.However he was not able to begin investigating immediately due to his military service in the First World War amongst other distractions.By the late 1920s Hubble was using the 100 inch Mount Wilson telescope, the largest in the world at the time, to look at distant galaxies.By 1929 he had catalogued 46 galaxies by their distance and speed (measured by the Doppler Effect).
The conclusions were unmistakable and confirmed Slipher’s results.Firstly most galaxies are redshifted, implying that they are receding from our galaxy, secondly, the degree of redshift (and therefore the galaxy’s velocity) is directly proportional to its distance away.In other words, if a galaxy was twice as far away as another, it would appear to be travelling twice as fast.This is now called Hubble’s Law, and it made his name.Soon after he was being referred to as “the world’s greatest astronomer”.Hubble’s Law opened up a new way of determining intergalactic distances.If you measure the redshift of a galaxy, you will know its velocity, and by Hubble’s Law, you will automatically know its distance away.
Why are the galaxies receding?
Other galaxies are not moving away because they are recoiling in disgust from the Milky Way, but appear to be receding because the Universe is expanding. The very fabric of space is stretching, carrying the galaxies with it. Observers in any galaxy will see every other galaxy as redshifted, so there is nothing special about the Milky Way. There is another important consequence of the fact than the Universe is expanding. If the Universe is expanding today, that implies that in the past the galaxies would have been closer together and the Universe was much smaller than it is now. Further back in time there would have been a time when they were squeezed closer together still and ultimately there would have been a time when everything was at a single point. Today there is extensive evidence that the Universe burst from a massive explosion, the “Big Bang”, some 13.8 billion years ago and has been expanding ever since.
We live in an expanding cosmos, let us with a look at the wider Universe and how our galaxy fits into it.We are not alone.Galaxies congregate as their enormous gravitational pulls slowly draw them together, locally defying the expansion of space.This happening to both our own galaxy and the much larger Andromeda Galaxy, both are hurtling together and will in fact collide in the distant future.The two galaxies are expected to absorb each other and merge into a new gigantic galaxy.This has probably happened many times before, the Milky Way contains remnants of other smaller galaxies it has consumed over the aeons.
The Milky Way and the Andromeda Galaxy are the two largest members of a collection of 35 galaxies, imaginatively called the Local Group.This dumbbell-shaped gaggle of galaxies is spread over 10 million light years of space.The Local Group is itself part of the much bigger Virgo Supercluster which contains about 100 groups and clusters of galaxies spanning more than 100 million light years.Yet we are nowhere near the end of the story, the Virgo Supercluster is just one of many such gigantic swarms of galaxies.
Large scale maps of the cosmos show it to be uneven, with great spongy concentrations of superclusters of galaxies brought together by gravity separated by huge voids of almost completely empty space.
In the next decades as telescopes improve we ought to get an even clearer idea of our place in the Universe.The great astronomers of the past who laid the foundations on which our knowledge of the Universe has been built would be astonished, awed and delighted by what the astronomers of today have found.