Institute of Astronomy


Ask an Astronomer - Structure of the Universe

End of the Universe

Published on 07/11/2012 

Many astronomers predict that the universe will continue expanding till eventually all life will end the stars will go out and the universe will be cold and lifeless.
If gravity is the strongest (as well as the weakest, we can all pick up a pen) force known, why does everything have to end.
The astronauts showed, with the particles in the bag, that even in a vacuum objects with mass attract, so with this in mind, why won't the universe eventually slow to a stop and then contract to its starting point and a new big bang happen.
Is this possibly what has been happening since before this time began?
Matter cannot be unmade apparantly, only changed into something else, so even with infinite distances even the minutest amount of gravity would attract a smaller object to a larger one even at a molecular level, surely.
I would be interested in your comments, although I am not an astronomer or otherwise involved in space science, I am interested to find out if this has been put forward by greater minds than mine and if so who, when and where can I read about it?

If the Universe only contained matter then the expansion of the Universe would indeed gradually slow down due to the gravitational attraction and eventually start to re-collapse down to a 'Big Crunch', the opposite of the Big Bang.

The fly in the ointment is what we currently call Dark Energy.  Astronomers and physicists are sometimes not the most imaginative of people, so rather like with Dark Matter we tend to just call something 'Dark' if we can't see it and don't know what it is.  Dark Matter is essentially just matter that we can't see, it still interacts through gravity in just the same way as normal matter.  Dark Energy on the other hand is different, rather than attract things together Dark Energy drives them apart and accelerates the expansion of the Universe.  There are an extremely large number of ideas about what Dark Energy could be but none of them are obviously any more likely than the others and indeed some physicists are unconvinced whether it really exists at all, though a lot more think it does than doesn't.

Anyway, the eventual fate of the Universe is down to the balance between Dark Energy and matter (including Dark Matter), and to what exactly the Dark Energy is, so there are various different scenarios:

  • One option is the Big Crunch I mentioned earlier, depending what the Dark Energy is and how it behaves, this could still happen, though it currently seems less likely.
  • The one that you were asking about is commonly referred to as 'Heat Death', which is essentially that if the Universe continues expanding forever then eventually all sources of stellar fuel will be exhausted, so all stars will eventually go out, and after long enough the Universe reaches a uniform temperature near absolute zero.  The subtly here that makes this a problem for life is that any form of life requires temperature gradients in the background, so if there are none that is a problem.
  • Another option is what is known as a Big Rip, which happens if the rate of expansion accelerates indefinitely.  In this scenario the Universe is eventually expanding so fast that it rips apart, first galaxies and stars, and then eventually molecules and atoms.

There are variations on these three broad scenarios as well depending on the flavour of Dark Energy used.

As there is currently no clear idea of Dark Energy might be there is also no clear idea of exactly what the eventual fate of the Universe will be, however a Big Crunch scenario does seem less likely.

Observing the early universe

Published on 24/09/2012 

How do astronomers observe the beginning of the universe: what wavelengths do we use, and what instruments are best for these measurements?

The early universe is of great interest to astronomers and cosmologists. It is also a popular topic amongst the general public, as we like to know where we came from. Astronomy has typically relied on light, that is electromagnetic radiation, for observing the universe. Electromagnetic radiation comes in a range of wavelengths, or equivalently frequencies, of which familiar visible light is just a small part. The spectrum goes from gamma rays at the highest energies, smallest wavelengths and highest frequencies, through X-rays, ultraviolet light, visible light and infrared to radio waves at the lowest energies, longest wavelengths and lowest frequencies.

The most useful for observing the early universe are microwaves (part of the spectrum of radio waves). We can measure the cosmic microwave background (CMB), which is the oldest light still in existence. Before this was emitted, the universe was effectively opaque: light couldn't travel freely because it was so hot and dense. Our cosmological models predict that the light that makes up the CMB was emitted about 400,000 years after the Big Bang. So we can't quite see the beginning of the universe, but it is close!

The CMB has been observed using many instruments. It was first discovered using radio receivers on the ground by American astronomers Penzias and Wilson in 1964. They would win the 1978 Nobel Prize for this. Lots of exciting science has been done more recently following a number of space-based missions. There was COBE launched in 1989 (the team won the 2006 Nobel Prize), then WMAP launched in 2001. The most recent mission is Planck, from which the data is still to be released (due January 2013). This will produce the most detailed map of the CMB sky yet.

While the space based missions are best for entire sky surveys, it is possible to measure parts of the sky from Earth. There are a number of ground-based and even balloon-based detectors. One telescope, the Arcminute Microkelvin Imager (AMI) is based here in Cambridge.

The CMB is currently our best tool, but it may be possible in the future to see further back in time. For this we would need to shift from using electromagnetic waves to gravitational waves. Gravitational waves are predicted by Einstein's theory of general relativity, they are tiny ripples in spacetime. We know they exist from careful observations of binary pulsars (for which Hulse and Taylor won the 1993 Nobel Prize). However, we have still to detect them directly.

We have ground-based detectors (LIGO in America, and Virgo in Europe), that are currently being upgraded. Japan is also in the process of building a detector, KAGRA. It is hoped will make the first direct detection in the next few years. However, these are unlikely to see gravitational waves from the early universe - just like electromagnetic radiation, gravitational radiation comes in a range of frequencies. We might be able to detect the gravitational background using pulsar timing arrays. Radio telescopes watch the signals emitted from a network of pulsars across the sky and look for correlations in deviations from their regular signals. The difficulty with these measurements, apart from having to make very careful timings, is that the frequencies are so low, you have to wait for years to see an entire wave. It is hoped that the planned Square kilometre Array will make these measurements easier.

Another possibility would be a space-based detector. This would be sensitive to frequencies between pulsar timing arrays and the ground-based detectors. Amongst many other sources, it could detect a gravitational wave background. Unfortunately, there is no funded space based detector mission at the moment. There may be eventually a detector based upon the design of the  Laser Interferometer Space Antenna (LISA). The European Space Agency is launching LISA Pathfinder in 2014 to test the technology needed, and hopefully a full mission shall follow that.