Australian Sky & Telescope — July 2017

20 AUSTRALIAN SKY & TELESCOPE July 2017

œAFTER THE VIOLENCEof the Big Bang

fully subsided, some 370,000 years later,

the universe was filled with cool, neutral

hydrogen gas. But those cosmic dark

ages came to an end when the energetic

radiation of the first stars and quasars

started to ionise their surroundings. Over

time, these growing bubbles of hot gas

became ever more numerous. Eventually,

they started to overlap each other and,

in the end, intergalactic space became

ionised.

Astronomers can learn a lot about the

early evolution of the universe by studying

this Epoch of Reionisation (it’s called

re-ionisation because the gas started

out in an ionised state right after the Big

Bang). Neutral hydrogen atoms naturally

emit at a radio wavelength of 21.1 cm

(a frequency of 1420.4 MHz), but the

radiation from these very early epochs

has been redshifted by the expansion of

the universe to wavelengths of a couple

of metres. This range falls in the low-

frequency regime. By mapping the sky at

various wavelengths (corresponding to

various redshifts and look-back times),

cosmologists will be able to reconstruct

the details of how neutral hydrogen

disappeared from the scene.

370,000 0.1 billion 1 billion

Years after the Big Bang

Redshift

Reionisation

Fully ionised Fully ionised

4 billion 8 billion 13.8 billion

1100 9 0

Dark Ages

Formation of first

astronomical objects

Present day

Big Bang

Neutral

atoms form

PHOTO: GOVERT SCHILLING; ILLUSTRATION: GREGG DINDERMAN / S&T, SOURCE NAOJ

The Epoch of Reionisation

the white dishes of ASKAP are almost too bright to look at.

Technicians are using small mobile cranes to install new

phased array receivers.

Not far from the core region of ASKAP is the Murchison

Widefield Array (MWA), the third SKA precursor instrument.

More or less similar to LOFAR, it consists of 128 ‘tiles’ of 16

small, spider-like antennae each. By the time you read this,

the number of tiles will probably have grown to 256 — they’re

really cheap and easy to deploy. For SKA1-low, the antennae

will have a different design, and the tiles, or stations, will

be bigger and more numerous. Eventually, a total of 512

stations will be spread out over an area 65 kilometres across.

As each station will hold 256 antennae, the total number of

‘Christmas trees’ is 131,072. In the proposed second phase

of the Square Kilometre Array, the plan is to increase the

number of antennae to well over a million, and to have

outlier stations all over Australia and even in New Zealand.

Cosmic dawn

With 25% better resolution, eight times higher sensitivity,

MILESTONES IN COSMIC HISTORY After the Big Bang, the universe was filled with a soup of photons and subatomic particles. After 370,

years, it cooled enough for atoms to form, mostly neutral hydrogen. (This is also when the light of the cosmic microwave background was

released to fly freely through the universe.) The universe remained in a neutral state until light from the first stars and galaxies started to ionise

their surroundings in the Epoch of Reionisation. After several hundred million years, the gas in the universe was completely ionised.

SRADIO SPIDERS About knee height, the antennae of the Murchison

Widefield Array stand together in ‘tiles’ of 16. They cover a similar

frequency range as SKA1-low but with a different antenna design.

RADIO REVOLUTION