Dark matter revealed

I interrupt my surf week to draw attention to some great news – there is news on the Symmetry Breaking blog that strong evidence for Dark Matter has just been announced by NASA’s Hubble Space Telescope and Chandra X-ray Observatory, see the official announcement here. The evidence comes from observation of galaxy collisions, exactly as in the previous case of the bullet cluster collision (see DM posts below and below).

In the words of the official announcement.. “A powerful collision of galaxy clusters has been captured by NASA’s Hubble Space Telescope and Chandra X-ray Observatory. This clash of clusters provides striking evidence for dark matter and insight into its properties.The observations of the cluster known as MACS J0025.4-1222 indicate that a titanic collision has separated the dark from ordinary matter and provide an independent confirmation of a similar effect detected previously in a target dubbed the Bullet Cluster. These new results show that the Bullet Cluster is not an anomalous case”

Pic from http://hubblesite.org/newscenter/archive/releases/2008/32/image/a/

So much for the skeptics! However, it should be pointed out that the above experiment points to the existence of Dark Matter, not to its nature (what particles make up DM?). Hopefully, such info will be forthcoming from particle physics experiments such as the UK Zepplin experiment, or even the LHC at CERN.

P.S. official solution to Hubble puzzle next week (although several commentators have more or less got it)

Hubble puzzle

I’ll be away surfing in Biarritz next week, so I’ll leave readers with a puzzle to mull over. Nigel Cook’s comments on the post below reminded me of a slight problem I have with Hubble’s Law. The problem is laid out below: the challenge is for anyone to supply a straightforward answer in simple language (damned if I can).

As every schoolgirl knows, Hubble discovered that distant galaxies are moving away from us (or any other point) with a velocity that is proportional to their distance. This is the crux of the evidence for the expanding universe, not to mention a major piece of the evidence for the Big Bang.

The law arises from experimental observation and is usually written as

v = Hd

where v is the recessional velocity of a galaxy, d is the displacement of the galaxy from us and H is the Hubble ‘constant’, or the slope of the graph.

(Note that relativity predicts that it’s really space that’s expanding and the galaxies ride the wave, but this doesn’t affect the question coming. We can also ignore the fact that there is a correction factor for the time it takes light to reach us).

Every physicist reads this law as v1/d1 = v2/d2 =v3/d3 = H and it works fine. However, consider what Hubble’s Law says about any one particular galaxy. The equation clearly implies that the velocity v of a galaxy A (relative to some point) is proportional to its displacement d (relative to that point). But for non-zero velocity, the displacement d must be changing in time – therefore Hubble implies that the galaxy’s velocity is also changing in time – which is another way of saying that galaxy A is accelerating!

So there’s the puzzle: Does Hubble’s Law predict that distant galaxies are not just moving away from us, but accelerating? On the face of it, it does. If so, then a force must be acting. Hmm. Suspect the equation is misleading. After all, why all the fuss/surprise about the recently observed acceleration of the universe expansion? According to the logic above, it must be accelerating..

P.S. The question can be framed in terms of basic mechanics – surely any object that has a velocity that is proportional to its displacement it must be accelerating?

Cosmological distance ladder at Trinity College

I got back to Dublin just in time for a superb lecture on cosmology at Trinity College, hosted by Astronomy Ireland andThe Irish Times. The lecture‘The Cosmological Distance Ladder – the key to understanding the Universe’ was given by Micheal Rowan-Robinson, Professor of Astrophysics at Imperial College London. Professor Rowan-Robinson is extremely well-known for his contributions to the field of observational cosomolgy, for a classic textbook on cosmology, and for the asteroid that bears his name (to the public, he’s probably best known as the PhD supervisor of Brian May, the lead guitarist of Queen who recently returned to physics!).

As you might expect, the hall was packed. Luckily, I’d booked on the internet – when I arrived at my old physics department, there was a queue of people from the front door all the way up to the Schroedinger lecture theatre two floors above. The lecture started with an introduction to the activities of Astronomy Ireland by chairman David Moore. I found this very interesting – astronomy is probably the last bastion of the amateur scientist, i.e. the last area where amateur scientists can enjoy practising science and make an important contribution.

The main lecture was a superb introduction to cosmology, from a slightly unusual viewpoint. Professor Rowan-Robinson’s main theme was how all our models of the universe, right up to the today’s consensus cosmological model, have been shaped by the measurement of distance. Starting with the ancient Greeks, he outlined how the measurements of the diameter of the earth and the distances to the moon and the sun by Eratosthenos, Aristarchus and others led to early models of the universe (there’s a very nice description of this in Simon Singh’s book on the Big Bang). Moving on to Copernicus, Micheal explained that Copernican calculations of relative distances of the sun and planets were correct to 10% – a crucial breakthrough on the way to the heliocentric (sun-centered) model of the solar system (the stars have to be much much further away in a heliocentic model).

Another unusual point was the discussion of the first use of stallar parallax for distance measurement of stars by Bessel in 1838: in Micheal’s view, it was this evidence that really marked the death-knell of the earth-centered model. (Bessel’s data gave evidence for both the motion of the earth and the huge distances of the stars). Micheal then went on to describe the discovery of Cepheid Variables, stars that act as standard candles (Cepheids are pulsating stars whose period give a direct measure of their luminosity , and therefore their distance). He described how Cepheid Variables facilitated Hubble’s measurements of the distances to several galaxies, and combined with measurements of the velocity of the same galaxies (from their Doppler shift), led to the famous Hubble’s Law (v/d = H).

Hubble’s Law: the further away a galaxy is the faster its moving

Micheal then tied the experimental results in with relativity, explaining how Hubble’s Law agreed with the expanding universe model of Alexander Friedmann. He then described how the law led to the idea of the Big Bang and to an estimate of the age of the universe (1/H). Presumably due to time constraints, he didn’t mention a famous hiccup – Hubble’s estimates of galaxy distances turned out to be inaccurate, leading to an inaccurate estimate of the age of the universe, initially casting doubt over the BB model.

Micheal then moved on to today’s puzzles. He started first by giving a careful explanation of baryonic matter and dark matter (see post on Tim Sumner’s lecture on dark matter below). I was relieved to to see that Micheal was firmly in the dark matter camp and skeptical of MOND, quite different to Katherine Blundell’s stance at the Cambridge conference (see Cambridge cosmology post below). He then moved on to the observation of the accelerating universe from supernova measurements and the puzzle of dark energy (see post on dark energy below). He also explained the second source of evidence for dark energy, the flatness of the universe as evidenced by recent measurements of the cosmic background radiation. There was a great discussion of dark energy, the flatness of the universe and the implications for the age of the universe and cosmological constant.

The flatness of the universe and the accelerated expansion pose a great puzzle

The lecture finished with a discussion of the possible nature of dark energy (vaccum energy density) and a description of the ultimate fate of the universe. At question time, I asked the question students often ask me – is there a possible connection between inflation in the early universe and the current acceleration? Micheal’s answer: the feeble acceleration we observe today may in fact undermine current models of inflation in the early universe!

All in all, this was a fantastic lecture on cosmology, by a top practictoner in the field. There was a huge turnout and a great atmosphere, although I didn’t see many faces from Trinity Maths of Physics. Afterwards, the Professor came along with some of the organisers for a drink, and patiently answered yet more questions. A DVD of the lecture can be ordered (worldwide) on the Astronomy Ireland website ..

Diversion: Festival interceltique

I spent last week in France, at a folk music festival in Brittany (physicists have a life too). Originally a piping festival, the annual Festival Interceltique de Lorient is probably the largest celtic music festival in the world, with parades, concerts and performances from pipe bands, music groups, dance troupes from all the great celtic nations.

Le grand defile, Dimanche

The sheer scale of the celtic world could be seen from the number of delegations – from Asturias (Spain), Galicia (Spain), Brittany (France), Cornwall (England), Scotland, Wales, Ireland, Acadia (Canada), Australia and the Isle of Man. There were concerts every day in the afternoons and evenings, not to mention the Nuit Magiques, chereographed performances on a giant scale in the local football stadium – some say the Lorient Nuit Magiques were the inspiration for Riverdance.

Nuit magique at the Stade Moustoir

Some the most enjoyabe events were the smaller gigs in venues representing each celtic nation, from virtuoso Acadian violinist Dominique Dupuy to the local Fest-Noz (you can get a flavour of the Dupuy gigs on youtube here).

Dominique Dupuy in action with her band in the Acadien tent

On top of all this, there were sessions in some of the local pubs, with Irish, Bretons and others swapping tunes into the early hours (where yours truly comes in). The sessions were a treat for any musician, with tunes in Quay St orThe galway Inn, not to mention monster sessions with performers fresh from their gigs at the Pub Glen late into the night. This was the best part for me, as I enjoy playing music with musicians from slightly different traditions. I think folk music has an edge over other types of music when it comes to this sort of jamming – and if there is one thing better than a lively Irish session, it’s a session where there is a mix of cultures and traditions. Also, it’s very moving to hear a tune/song you’ve known your whole life played in a more minor, modal key – an older, deeper version that makes your version seem like a pale modern echo. (It’s less moving if some idiot is playing it on the bombard at 10 o’clock in the morning).

Fast tunes and sad songs with Brian Comb in Quay St

Yours truly has the last tune in the Pub Glen.Thanks to Gerard for the photos -you can see the full collection here

Early on in the week, a few of us were lucky to have a quiet afternoon session with some French Canadian musicians. It was only later we realised they were members of distinguished Acadien band Ode a l’Acadie. Sadly, accordion player Isobel Thierault seriously injured her foot the very next evening so didn’t see much of them for the rest of the week, although they gave a great concert at the Grand Theatre. You can find out more from the Ode a l’Acadie website and download clips of the band

Ode a l’Acadie

Overall, this is a great international music festival – a feeling of an inheritance that is shared, yet different. I’m constantly amazed at the sheer diversity of European culture and its effect on the world…there’s a nice discussion of this on the festival website.

What a great week…and nobody mentioned the problem of the cosmological constant once.