February 13, 2012
Cosmic Explosions, Dark Matter and the Fate of the Universe
Dr Mark Sullivan
Report by: Chris Sutcliffe
At the beginning of the meeting some important announcements were made. David Waugh mentioned the formation of a Radio Astronomy Group, details of which would follow in the near future. As Mike Dryland is leaving the committee, Mike Meynell has been appointed as vice-chair in addition to his existing responsibility for the website, social media and observation coordinator.
Sumitra spoke about the new Flamsteed Facebook and Flickr sites. Facebook includes details on forthcoming events, meeting write-ups and astronomy news. Flickr features astrophotography and anyone can submit photos if they set up an account. Details on camera settings for many of the photographs can be viewed. There also is a discussion board where questions can be asked and answered by members.
Jane then introduced Dr Mark Sullivan who is a Royal Society University Research Fellow at the University of Oxford.
He first introduced the subject of the accelerating universe. The universe appears static based on our visual observations. For example, the Hubble deep field image is only a fraction of the size of the full moon, with every object being a galaxy billions of miles away; but in 100 years, they will still all look the same. However, they are moving away from each other at huge speed and the universe is expanding with time.
In 1915, Einstein developed general relativity, one of the most successful theories ever developed. Einstein preferred a static model of the universe, in which space is neither expanding nor contracting, and he added a cosmological constant to his equations of general relativity to counteract gravity. He later called this his ‘biggest blunder’, after the discovery of an expanding universe.
Slipher in 1915-1917, and Hubble in 1929 observed the Doppler shift when observing distant galaxies. They noted that when light is passed through a spectroscope, the resulting spectrum includes dark absorption lines caused by the chemical elements in the galaxy. If there is a shift of lines to the red (redshift), the galaxy is moving away; if there is shift of lines to the blue (blueshift), the galaxy is moving towards us.
Slipher noted that most galaxies have redshift, and few have blueshift; therefore galaxies are moving away from us. Hubble discovered a rough proportionality of the distances of objects with their redshifts.
Mark then explained measuring distance using the standard candle technique illustrating this with the light from a supernova explosion reaching an observer on Earth. An object which is half the distance appears four times as bright and therefore if the you measure an object’s apparent brightness, you can determine the distance.
The most distant galaxies are moving away from us at faster speeds and Dr Mark presented a graph showing recession velocity in km per second against distance. Clearly, the universe is expanding.
Mark showed his Toy Universe of Galaxies where an image is stretched and overlaid on the original image. Moving from the centre, the more distant objects move faster; the distance between the closer objects is smaller than distance between the more distant galaxies. Galaxies further away are moving faster and there is no unique centre to the expansion.
THE UNIVERSE STARTS WITH THE BIG BANG
The universe expands as a result of the big bang. There are then two possibilities; either we have a ‘closed universe‘ where eventually the expansion energy is overcome by the gravitational pull of matter; or the universe expands forever (‘open universe‘) because the expansion energy is always greater than the gravitational pull of matter.
TYPE IA SUPERNOVAE
Tycho’s Supernova (SN 1572) was a supernova of Type Ia
Fritz Zwicky coined the term “supernova” and discovered around 120 supernovae, suggesting a type Ia supernovae as the standard candle for measuring distances in space. A type Ia supernovae is an explosion of a white dwarf star. The gravitational pull of the white dwarf star pulls sufficient matter from a nearby star to increase it’s mass beyond the Chandrasekhar limit.
This means that there is a standard amount of fuel to power the explosion, leading to a luminosity which is the same for all type Ia supernovae. From this standard luminosity, we can determine the distance to the supernova.
On 23 August 2011 a supernova was discovered in M101, 20 million light years away. The supernova went from non-detection to 17th magnitude in 24 hours, and then further increased in brightness every three days.
On the same evening, the Hubble Space Telescope was triggered, and the supernova soon reached magnitude 10. It was visible from the UK with small telescopes. This was the nearest type Ia supernova since 1986.
What did this supernova tell us? The supernova was found only 12 hours after the explosion (it actually happened over 20 million years ago!). Its luminosity tells us the size of the star which blew up; the luminosity measurement only works a few hours after the explosion. It was measured as less than 5% of the size of the sun, and confirms the theory that type Ia supernova originate from white dwarf stars.
No companion star was detected from Hubble Space Telescope imaging before the supernova exploded. This seems to rule out many of the single degenerative systems; it was not a red giant that caused the explosion.
In the 1990s, two teams used high redshift supernovae to measure how fast the universe is expanding. The supernovae were fainter than expected, and therefore must be further away, and it was concluded that something extra must be pushing them. The extra ‘something’ is dark energy. Einstein was not so crazy; he was right to introduce the cosmological constant, but for the wrong reasons.
Dark energy appears to be driving an accelerating universe, and a huge international effort is being made to understand dark energy. In an image of an area of the sky four times the size of the full moon, 0.5 million galaxies have been discovered, but this includes only 20 interesting supernovae, so being able to deal with huge volumes of data is the key to this research.
After showing a pie-chart illustrating that about 70% of the universe is Dark Energy, Mark said that nobody has the faintest idea what dark energy is!
WHAT IS THE ULTIMATE FATE OF THE UNIVERSE?
Mark presented an interesting graph plotting the scale of the universe from the big bang to the present with three curves. After the big bang there was a marked deceleration in the early period, followed by a steady acceleration over the passage of time to the present day.
Looking ahead, Mark demonstrated theories predicting either a steep acceleration (the big rip) where matter is torn apart by the expansion of the universe, or (if dark energy theory is incorrect) a sharp deceleration (the big crunch) with the universe contracting into a hot dense state, similar to the big bang.
Another scenariois the big freeze with the universe expanding and cooling forever, until the universe expands to such a point that heat death occurs (not enough heat to sustain motion).
WHERE NEXT?
This is one of the biggest unknowns in physics.
The Mount Palomar 48” telescope is looking for supernovae every night and generates a vast number of supernovae candidates, but this is a human recognition task (better than computers!) in which amateur astronomers can participate through the Galaxy Zoo.
All of us can help to identify the best candidates using a yes / no decision tree which is accessed via http://supernova.galaxyzoo.org/ After logging on, there is a tutorial showing how to take part, and the dates of the next observing run.
This was a brilliant lecture delivered with a very clear and structured approach which everyone found truly absorbing. Mark concluded by answering questions.
Chris Sutcliffe
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