Date: August 27th, 2012

Title: The Light Fantastic

Podcaster: Renée Hlozek

Links:Fermi telescope: http://fermi.gsfc.nasa.gov/
Chandra X-ray observatory: http://chandra.harvard.edu/
XMM-Newton: http://xmm.esac.esa.int/
Hubble: http://hubblesite.org/the_telescope/
SALT: http://www.salt.ac.za/
Herschel: http://www.esa.int/SPECIALS/Herschel/index.html
Atacama Cosmology Telescope: http://www.princeton.edu/act/
Planck Telescope: http://www.esa.int/SPECIALS/Planck/index.html
Square Kilometer Array: http://www.skatelescope.org/
Renée’s website: http://www.astro.princeton.edu/~rhlozek/index.html
Twitter: @reneehlozek

Description: As astronomers we use light to unravel the mysteries of the universe.
Renée Hlozek discusses ‘The light fantastic’ and provides some links to telescopes which operate across the electromagnetic spectrum.

Bio: Renée is a South African cosmologist working at Princeton University. She studies light originating from shortly after the Big Bang, and light from distant exploding stars. She is amazed daily at how amazing it is that we can decipher the universe using light (and physics, maths and reasoning of course!)

Today’s Sponsor: This episode of 365 days of Astronomy is sponsored by iTelescope.net – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.

Transcript:

Hi, my name is Renée Hlozek, and I’m a Lyman Spitzer Jr. Fellow in Astrophysics at Princeton University. I study cosmology, and I want to talk to you today just about a fact that you may not have thought about if you don’t follow astronomy news. This is actually something that I often think about and something that amazes me about astronomy and about our observations of astronomy.

All we use to infer everything we know about the universe at large: is light. That sounds like a really simple statement, but if you think about the way we typically think of science being done, we measure something, and often the idea is that there is something that you can repeat, but of course with light, all we get of course are the photons that come to the telescope at a particular time, and all our inferences on the temperature, mass, large scale structure and distribution of the universe, are made from those observations. So based on that, I thought that I would just briefly summarise a few facts about light and point you to some links and interesting thoughts so that you can think about these things yourself.

First of all, we know the electromagnetic spectrum covers a range of energies/frequencies and temperature. The first thing to think about is what  a blackbody is, which is an ‘idealised’ body, like a black box, that absorbs all radiation, regardless of the frequency of that radiation. So if you turn that onto its head (so to speak), a black body that is radiating in thermal equilibrium radiates electromagnetic radiation that is only defined by the temperature of the body itself. So the spectrum, or the change of intensity of the light as a function of frequency, is uniquely determined by the temperature of the body. And we use this term ‘black body’ to describe different objects in the universe in terms of their temperature.

We measure light in a range of frequencies and with a range of instruments, and I’m just going to briefly go through a list of these so that you can investigate further. So if we start at the energetic end, gamma rays are very, very energetic rays, and they have a wavelength of 10^{-12} m, so very short wavelength and very high energy. One of the telescopes used to probe this radiation is called the Fermi Telescope, and this probes supermassive black holes, neutron stars and accelerated jets of gas, so anything that has a lot of energy output in the universe – we use gamma rays to probe.

Moving slightly further down the spectrum, we have X-rays. X-rays also measure hot, energetic particles in the universe. An example of something that we would study with X-rays are galaxy clusters and specifically the distribution of hot electron gas inside the centre of these dense galaxy clusters. For example, telescopes such as the Chandra X-ray observatory or XMM-Newton will probe this radiation. Typically gamma ray and X-ray telescopes are space-based telescopes.

On the UV side, currently (to my knowledge) there isn’t a UV telescope, but ROSAT, that is short for the Röntgensatellit is a telescope that measured X-ray but also UV radiation, again to look at hot objects, and energetic objects in the universe.

As we move down to the visible regime, we have optical telescopes. A key example of an optical telescope of course is the Hubble telescope, but also the South African Large Telescope (SALT) measures optical light. Telescopes in the optical regime take pictures (or measurements) in the way that we see with our eyes, although telescopes can of course “see” much fainter.

But the (EM) spectrum continues and if we move to longer wavelengths we have infrared (IR) telescopes such as the Herschel telescope, which measures wavelengths of about 10^{-5} m. Now, the key idea with infrared telescopes is that they are typically used to measure dusty regions in the universe, which we can’t see with optical telescopes because they block out visible light, just as a smoke stack  blocks out the view. IR telescopes measure re-emitted radiation from these dust particles, and so images that previously look dark in the optical bands glow in the IR. Herschel was launched a few years ago, and is really changing the field of IR astronomy by providing us with high-resolution images.

In microwaves, my favourite telescope to talk about is one that I work on, which is the Atacama Cosmology Telescope (ACT), which works at cm wavelengths. ACT measures the temperature of the Cosmic Microwave Background, which is light that has been travelling towards us since shortly after the big bang, a few hundred thousand years after the big bang. ACT measures the temperature of that radiation as a function of position on the sky.

Finally, as you move down to longer radio wavelengths, these are wavelengths which are very long (up to 1000 m), projects like the Arecibo telescope, interferometric arrays that we have around the world (VLA, VLBA, VLTI, ALMA etc.) and of course everyone is looking to the future with the Square Kilometre Array (SKA). The reason why you can use an interferometer array is because the signals from these radio telescopes are actually measured as electronic signals rather than light images in an optical telescope. So you can connect different telescopes together and use signal processing  techniques to make images of the sky.

Now, you have heard me talk about resolution. The way we measure the (possible) resolution of the telescope is by taking that wavelength of the light that is going to be measured and dividing it by the
diameter of the telescope. A similar thing also  works for radio telescopes where you divide it by the baseline between two telescopes. And so, in this way you can measure (determine) what kind of objects you will be able to distinguish, for example with a low resolution you won’t be able to make out the centre of a galaxy, rather you will see one blob where the galaxy is, however with a finer resolution you would start to make out structure, filaments and parts of the galaxy.

I recommend you take a look at the websites of the telescopes I discussed (given below), as they contain a lot of information on the science case of the telescopes, and information on how you can look at the images and press releases on new astronomical data.

Another thing that is really important is the fact that as we look at fainter and fainter objects, we are actually looking back in time. We know that light takes a finite time to travel towards us, it travels with the speed of light, of course. And so, we are actually looking into the past – stars that we are looking at today are no longer there, because we are looking at them as they were billions of years ago.
There is always a continuous race to find the oldest galaxies, one of the oldest galaxies in the universe, UDFY38135539, is a galaxy that has been found at a redshift (z) of 8.6, so light has taken 13.1 billion years to reach us.

You heard me mention the term redshift. Redshift (z) is a concept in General Relativity, that we use to describe objects. It is an observational quantity that we measure by comparing the wavelength of an observed feature, such as a Hydrogen line in the spectrum of a galaxy to the frequency or wavelength that we know that feature must happen at. For example, Hydrogen emits lines (radiation at characteristic frequencies/wavelengths) at a very specific wavelength in the rest frame on earth, and so that’s what we use to determine the change in wavelength: we divide that by the wavelength itself and we come up with a quantity called redshift.

[The equation is z = (WL_observed – WL_emitted)/WL_emitted ]
Redshift is connected to distance (in the universe) and connected to time, and so as I said the oldest galaxy that has been measured is roughly at a redshift of 8.6, the oldest object that we have observed is of course the Cosmic Microwave Background (CMB) radiation, which is at a redshift of roughly 1000: 1089, which means that we are looking at light that has been travelling to us from very shortly after the big bang.

I hope I interested and excited you about the fact that we only get light, and all our observations are made using light and its properties and that we spend all our time  trying to observe as many photons as we can. I hope you will think about that the next time you point you camera at a friend, or look up  at the night sky.

Thank you!

End of podcast:

365 Days of Astronomy
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