After a fortnight gallivanting around Europe and being more creative in my modes of transport than expected thanks to an unpronounceable mountain in Iceland, I’m back in Hawaii. On the flight to LA I ended up chatting to a bloke who works for a large US computer firm about various geeky things. The “what do you do?” question came up, and given he seemed worth talking to I opted for astronomer rather than physicist. Then at a lull in the conversation he volunteered the question, “how do they know how old the universe is?” We’ve been planning to add a “How do we know?” category to the blog so this seems like the perfect place for me to start.
The simplest measure of the age of the universe is known as the Hubble Time. The universe is expanding, we know this because we can see that light from distant galaxies is Doppler Shifted towards redder wavelengths, indicating they are moving away from us. The further away the galaxy, the faster it moves away. This is known as Hubble’s Law. The rate at which the recession velocity of a galaxy increases with its distance for us is known as the Hubble Constant. If we know how fast the universe is expanding, we can extrapolate back and see when the universe would have a size of zero, ie. when the big bang happened. Of course if we know how long ago the big bang was, we know roughly how old the universe is.
So all we need is the Hubble Constant, easy yeah, erm not really. The value of the Hubble Constant was for half a century the subject of great dispute. This period was known as the Hubble Wars which conjures up massed ranks of Welsh longbowmen cutting down the flower of French chivalry to establish domination over the fundamental constants of the universe. In reality it was a debate about measuring the distance to far off galaxies. Getting recession velocities of galaxies is pretty easy, but to find the Hubble constant, you’ve got to know the distance of each galaxy too. Measuring distances in astronomy is pretty hard, something we might deal with later in this series, so various novel techniques must be used.
When Edwin Hubble first worked on the recession velocities and distance of galaxies in the 1920s and 30s he used a fortuitously odd type of star as a “standard candle”. In astronomy, if you know how much light a star or galaxy puts out in total and how much we receive on Earth, you can combine these to get how far away it is. Hubble used unstable stars which have finished their main life as a normal star, known as Cepheid variables. They pulsate, and so vary in brightness, and the really lucky bit is that the pulsation rate is related to the total light emitted by the star. So the pulsation period gives the total light emitted – combine this with the apparent brightness and you get the distance. The problem is that you need a pretty powerful telescope to resolve individual stars in distant galaxies. Even with the largest telescope available in the middle of the last century, the 5m Hale Telescope on Palomar, only relatively nearby galaxies can have their Cephieids resolved from the mass of other stars. So astronomers had to get creative.
This doesn’t mean they went off and played guitar in Queen, Coldplay or, (as rumoured in the case of one astronomy blogger) the opening act for The Velvet Underground. Science itself is a creative process, trying to dream up innovative solutions to work around the limitations of the available technology and data. The work mostly rested on calibrating a myriad of new distance indicators using local galaxies at known distances (such as M100, pictured) and applying these new estimates to more distant objects. The results fell into two broad camps, one side led by the American astronomer Allan Sandage claimed a value of about 50 (I won’t go into the slightly obtuse units used for this measurement) and another led by the French cosmologist Gerard de Vaucouleurs claimed a value of about 100. For decades they fought over seeming minor points that shifted one particular rung on the intricate astronomical distance ladder up or down. From how dust in our own Galaxy affects the measured brightnesses of distant galaxies to subtle biases in samples of galaxies to the brightness of exploding stars, no point in the other group’s work was too minor to pick apart.
Fast forward to the end of the last century and say hello to the now 20-year-old Hubble Space Telescope. One of its key projects was to pick up where its namesake left off and find the Hubble Constant using Cepheid variables in more distant galaxies. After a huge amount of effort it came out with a result of about 72, giving an Hubble Time of roughly 13.8 billion years. This fits in fairly well with the estimated ages of the oldest stars. More recent measurements, such those from the WMAP study of ripples in the cosmic microwave background and more up to date supernovae studies have supported a value of roughly 70. However they also predict the expansion of the universe is accelerating, meaning our simple extrapolation, assuming constant expansion won’t give exactly the right answer.
I didn’t say all this to the bloke on the plane, we were about to land so I didn’t have much time, but I hope I got it across fairly well both to him and you.
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So Emma got there first, straight to my all time favourite Hubble moment. Then Stuart stole my second favourite! You know.. great minds..
But nevermind, because with Hubble everything comes a close 2nd, or 3rd, or 1st. But here’s an image that has been making its way into my presentations since the very first time I saw it:
It’s disk galaxy NGC 5866, which faces us pretty much edge on. I love the way this image shows a galaxy from a slightly less known perspective. You can see the striking dust lanes going along the disk, and the slightly yellowish bulge of older stars in the middle – just beautiful. And if you can, for a moment, take your eyes off NGC 5866 you can see a number of galaxies in the background and it always amazes me how many you can see. I can have endless fun with the zoomable version of this image – go on, have a go and have a good look around.
And I do mean a good look around. Spare as much time as you can. Because every one of those galaxies has such a complexity to it, such a history and a future that it almost pains me to dismiss it as ‘seen’ at any given moment. There are billions of stars there, with numerous planetary systems and civilisations that almost certainly surpass our imagination; endless physical phenomena that we don’t yet understand or probably even know exist; and – I like to think – endless renditions of the blues. This stuff is real and sometimes, for as beautiful as the Hubble legacy is and for all it has done for us, I can’t help thinking that no image and no finite amount of staring can ever do this Universe any justice.
Image credit: NASA
Happy birthday Hubble ! Thank you for many years of stunning views of the Universe.
I have to say Emma pretty much took the best image Hubble has ever taken as her favorite. Its a hard act to beat ! I am however very partial to this series of images taken by Hubble over the years.
As the glorious Hitchhikers Guide to the galaxy tells us “Space… is big. Really big. You just won’t believe how vastly hugely mindbogglingly big it is. I mean you may think it’s a long way down the road to the chemist, but that’s just peanuts to space…”. A lot of things in space are big as well and that normally means that the time scales over which they change are very long. So long in fact that its often very hard to see things changing at all, the universe appears timeless. What I love about these images of the variable star V838 Monocerotis is that over only a few years we can see the image of the star changing.
The star at the center of this image underwent a catastrophic outburst. In a short period of time it became roughly 1 million times brighter than our sun, giving out a huge amount of light in the process. The light which came directly towards us arrived first but then gradually over the years, light heading away from the star in a shell, reflected off of surrounding gas and dust eventually heading towards the earth. As we let time go on the light illuminates different regions around the star.
Despite the fact that the images look like they are showing a concave bubble around the star its actually convex with the material which appears to the the side of the bubble closer to us actually being behind the star.
I just love seeing something in the sky evolve in this way! Thanks again Hubble !
Twenty years ago today the Hubble Space Telescope was launched. Putting a telescope in space gets rid of all the distortions that come from our atmosphere, and means the images it takes are much sharper than equivalent instruments on Earth and, after a few initial teething problems, Hubble has demonstrated this magnificently, not just for astronomers but for the general public as well, through the excellent Hubble Heritage Project which turns data into art. Hubble has maintained its world-leading position because, unlike the new generation of space telescopes such as Herschel, it is close enough to get regular servicing missions from the Space Shuttle fleet.
In celebration of its 20th birthday today I thought I’d post one of my favorite Hubble images, the Hubble Ultra Deep Field.
This immensely deep image is of a tiny area of the sky in the Fornax constellation – its only a tenth the diameter of the full moon – and it took Hubble nearly a million seconds (though not all at once) to make it. Part of the observations were taken over Christmas 2003/2004 so maybe the astronomers wanted to leave it doing something useful while they went off to celebrate! Nearly everything you can see in this image is a distant galaxy, there are roughly 10,000 visible. More than anything else it was this image, and the Deep Field that preceeded it that really brought home to me how vast the Universe is, and how much there is for us to learn about it. I find it simply amazing.
If you want to take part in research using data taken with Hubble, the hundreds of thousands of archive images have just been incorporated into Galaxy Zoo for people like you to classify. From everything we’ve seen from the telescope so far, who knows what you might find!
Picture Credits: NASA
Wow, just wow. This is the Sun as seen by the recently launched Solar Dynamics Observatory (SDO). It’s actually a combined image, showing several different wavelengths at once (blues and greens represent hotter regions than reds). The SDO is the most advanced solar spacecraft ever launched, and it shows in the level of detail visible in this picture. Just look at all that material streaming out into space; ‘space weather’ like that can seriously disrupt satellites. Understanding what’s going on inside the Sun, and therefore predicting the most violent solar events, is one of the main aims of the SDO mission.
Luckily, on 30th March, not long after the SDO instruments were switched on, the Sun obliged those of us who like spectacular first light images (and I think that probably includes everyone) by throwing off a giant solar eruption. And here it is:
(if you can’t see anything there click here…)
I love the sense of scale you get when the movie zooms out to show the whole disc.
Oh, one final thing, even the SDO’s launch produced great pictures when it destroyed a sundog. It was clearly a good omen for the mission!
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I read 3 papers! Three beautiful, science-packed, revolutionary and mind-blowing papers! Ok, maybe not – but trust me, after being so busy with things like measuring redshifts, fixing codes and mountains of admin/conference organising, almost any science is pure beauty for the old brain.
But one of these papers did tap into something I’m quite interested in, and it’s related to the Cosmic Microwave Background (CMB). I’ve briefly mentioned the CMB before, but here’s a more decent introduction: when we say CMB we are talking about radiation that was created when the Universe was very very young – around 300,000 years old. At that time the Universe was hot and radiation (or photons) were the dominant component of the Universe. Because it was so hot at that time, the Universe was actually opaque – matter was ionized, meaning electrons were bobbling about not really being attached to any nuclei because of the high temperature. What this means is that photons could not get very far without bumping into something – they could not travel in a straight line for any decent sort of time, and were perpetually scattered around. That’s essentially what opaque means. However, something special happened around 300,000 years into the Universe’s lifetime, and that was a decrease in temperature that allowed these electrons to settle into atoms, effectively setting the photons free. We call this the time of last-scattering.
These photons were then free to travel unhindered. They were travelling then, and they are still travelling now! Straight into our telescopes, carrying information from around 13 billion years ago, virtually unaltered! This, for Cosmologists, is like Christmas – almost too good to be true. Anyway, I say virtually because some things affect these photons as they travel along. A big one is the fact that the Universe is expanding, and this causes their frequency to change. This is the reason why we see them in the Microwave part of the spectrum today – they were a lot hotter 13 billion years ago. Had we lived earlier on during the lifetime of the Universe, the CMB would have peaked in the visible band and the sky would be rather pretty! (of course, whether a planetary system with life could even exist then is another matter) But the Universe, in spite of being mostly empty, still has a lot of stuff around. You know, galaxies and things. And as these photons go through expanding regions with more matter (clusters) or less matter (voids) they see their frequency slightly changed.
Now, what I didn’t say and should have done is that not all CMB photons are the same. They are all very similar, but depending on the density of the region of space they were in at the time of last-scattering, today they are either a little bit hotter than the average, or a little bit colder. It’s a very small little – around 1 part in 10,000! But as I said before, a good enough experiment is able to pick up these tiny differences. When we look at the sky, we obviously only see the CMB photons travelling towards our telescope (they are travelling uniformly in every direction), but these tiny differences are projected in the sky in what is now a very familiar pattern, and that I need to show time and time again because it really is beauty:
What you’re seeing here is the whole sky, as seen in the microwave band (after having radiation from our own galaxy cleverly removed). Red patches are slightly hotter photons, blue patches are slightly colder photons. These patches tell you about the density distribution in the early Universe. It’s these fluctuations that seed the density fluctuations that give birth to stars and galaxies and you and I, but I’ll leave that to another time.
Right now, I want to focus on a small patch of these fluctuations. In you look at the bottom right corner rim, around 4:30pm if that map was a clock (oooohhhhhh!! That’s an idea! Can I have a CMB clock anyone?), you’ll find a cold patch that has been nick named The Cold Spot (one day I’ll write a rant about how Astronomers, being a rather clever and creative bunch of people, are rubbish at coming up with good names for things. mm.. maybe I just have.). Now, by eye the Cold Spot doesn’t look any different from other cold spots around the map. However, statistically, and given our model for the early Universe (which describes things pretty well, including the distribution of galaxies we see today), a spot of that shape, size and temperature has a very low probability to exist. ‘Very low’ here means something around 0.1% to 5%, depending on different estimates. This is slightly uncomfortable and Astronomers have spent a significant amount of time studying the Cold Spot.
There are a few options here:
1) The Cold Spot was formed at the last-scattering.
2) The Cold Spot was formed during the photon’s path to us.
3) The Cold Spot is an instrumental artifact.
4) The Cold Spot is a data-reduction artifact.
There are papers studying all of the above. 3) and 4) seem unlikely at this stage but should not yet be discarded. Having data from another experiment like Planck, with a whole new reduction pipeline, will help. 1) is fascinating because it could potentially mean there is something amiss from our early-Universe theory. But the paper that prompted this ridiculously long post actually focuses on 2).
Bremer et. al investigate the hypothesis that actually, the reason why we see a particularly cold spot in that region of the sky, is because there is a large void (a region of space with much less galaxies than the average) along that line of sight. The trick here is to note that the Universe is expanding – i.e., the shape and size of clusters and voids changes with time. The frequency (or temperature) of photons is affected by this change because the energy they loose or gain when they go into these structures is not completely recovered when they come out. There is a net effect, with is to gain a little bit of energy going through clusters, and loosing a little bit going through voids. There’s a little animation here that may make it clearer.
So Bremer and collaborators chose 5 regions of the sky, all inside the Cold Spot, and took redshift surveys in these 5 regions. This allowed them to see how the distribution of galaxies changed with distance between us and the region of last-scattering, and they looked for a deficit of galaxies that would be significant enough as to imprint the Cold Spot in the observable CMB. They did this by comparing these redshift distributions with those from other regions of the sky (outside the Cold Spot) and did not find any sign of such a deficit, or void. They could only look up to redshift of 1 (or around 7.8 billion years ago) because of instrumental limitations, and they didn’t have enough galaxies below redshift of 0.35 (around 3.8 billion years ago), but they covered a significant chunck of time when this effect is more likely to happen and did not find what they were looking for. What this means, is that they discarded at least some theories that could lead to point 2), although not all of them.
So the jury is still out on the Cold Spot. Personally, I’m sometimes tempted to add a 5th option:
5) The Cold Spot was formed at last-scattering, but its significance is being over-estimated.
But that, I’m afraid, is another post (I’m late for work!!).
M. N. Bremer, J. Silk, L. J. M. Davies, & M. D. Lehnert (2010). A redshift survey towards the CMB Cold Spot Submitted to MNRAS arXiv: 1004.1178v1
Having been stuck in the Netherlands due to the eruption of Eyjafjallajökull I’ve finally managed to escape, if only to the UK. While this was nowhere near as impressive as my Escape from Belgium during a total public transport strike it was still a bit stressful. I’ve just had a chance to check the blog and found that Emma’s post from last year on Volcanoes from Space has become quite popular. Hence I had a bit of a dig around and found that the NASA Earth Observatory has a few amazing images of the ash cloud drifting out over Europe. If you are still stuck feel free to shake your fist at the screen while looking at these.