The Naked Scientists-June 2012

The first lecture was given by Dr Dominic Ford from The Cavendish Laboratory in Cambridge.

The lecture was about the Square Kilometre Array (SKA) and what radio astronomy reveals about the universe.

The Square Kilometre Array is expected to take 5 years to be built with initial observations expected in 2019.

The array will be located in two sites: South Africa and Australasia (sparse aperture array). 70 MHz to 370MHz signals will be received in Australasia and GHz signals will be received in South Africa.

Radio telescopes need to be very large due to the long wavelengths of radio waves.
One of the aims of the telescope is to image the night sky, looking at stars, galaxies and the Large Magellanic cloud (a companion galaxy to our Milky Way). There are believed to be 1xE22 stars in the universe with an average stellar mass of 2xE30kg. Altogether there is believed to be 1xE50kg of “stuff” in the universe. Material clusters are not the simplest distribution of matter.

The Cosmic microwave background radiation is the remnant of the big bang. WMAP (Wilkinson Microwave Anisotropy Probe) has been able to look at the universe 500000 years after the Big Bang.

Immediately after the Big Bang the universe was smooth. Matter was evenly distributed.

The 2dF Galaxy Redshift Survey showed the structure of the universe formed very early on after the Big Bang.

Another aim is to investigate the “Dark Ages” of the universe. This is difficult because there is no light. The European Extremely Large Telescope (in about ten year’s time) will hopefully be able to help do this.

Different wavelengths of the electromagnetic spectrum can be used to view different parts of the universe. Hydrogen is detected from radio waves. Infra-red can show up dust.

The 21cm hydrogen emission line is especially useful. In radio astronomy the spectral line appears within the radio spectrum (in the microwave window to be exact). Electromagnetic energy in this range can easily pass through the Earth’s atmosphere and be observed from the Earth with little interference.                                                                                                                                                     Assuming that the hydrogen atoms are uniformly distributed throughout the galaxy, each line of sight through the galaxy will reveal a hydrogen line. The only difference between each of these lines is the doppler shift that each of these lines has. Hence, one can calculate the relative speed of each arm of our galaxy. The rotation curve of our galaxy has also been calculated using the 21-cm hydrogen line. It is then possible to use the plot of the rotation curve and the velocity to determine the distance to a certain point within the galaxy.

Hydrogen line observations have also been used indirectly to calculate the mass of galaxies, to put limits on any changes over time of the universal gravitational constant and to study dynamics of individual galaxies.
The line is of great interest in big bang cosmology because it is the only known way to probe the “dark ages” from recombination to reionization. Including the redshift, this line will be observed at frequencies from 200 MHz to about 9 MHz on Earth. It potentially has two applications. First, by mapping redshifted 21 centimeter radiation it can, in principle, provide a very precise picture of the matter power spectrumin the period after recombination. Second, it can provide a picture of how the universe was reionized, as neutral hydrogen which has been ionized by radiation from stars or quasars will appear as holes in the 21 centimeter background (Too hot and neutral hydrogen doesn’t occur and too cold, hydrogen forms molecules).

However, 21 centimeter experiments are very difficult. Ground based experiments to observe the faint signal are plagued by interference from television transmitters and the ionosphere, so they must be very secluded and careful about eliminating interference if they are to succeed. Space based experiments, even on the far side of the moon (which should not receive interference from terrestrial radio signals), have been proposed to compensate for this. Little is known about other effects, such as synchrotron emission and free-free emission on the galaxy. Despite these problems, 21 centimeter observations, along with space-based gravity wave observations, are generally viewed as the next great frontier in observational cosmology, after the cosmic microwave background polarization.

The Very Large Array, one of the world’s premier astronomical radio observatories, consists of 27 radio antennas in a Y-shaped configuration on the Plains of San Agustin fifty miles west of Socorro, New Mexico. Each antenna is 25 meters (82 feet) in diameter. The data from the antennas is combined electronically to give the resolution of an antenna 36km (22 miles) across, with the sensitivity of a dish 130 meters (422 feet) in diameter. It has to be this big for the resolution to match the wavelength. This set up would take 100 years to investigate the dark ages which is why we need the Square Kilometre Array.

The Square Kilometre Array dishes will not be arranged regularly. Separation is important. A spread of 0 – 3000km gives varied information. The dishes have to be placed well away from mobile phones. Images will be cleaned up easily – discrete sources of noise such as planes will easily be subtracted. The array will provide an Exabyte of data a day. Hopefully it will allow us to see 10 billion years in the past of the universe. We have theories about how galaxies form. If we can see the dark ages this would confirm these and give us a better understanding of dark matter.

In the beginning the universe was smooth until dark matter got going. The Cosmic Microwave Background shows us the transition from ionised hydrogen gas (which can’t be compressed) to neutral hydrogen (which can be compressed) forming gravity wells and the universe developed structure.


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