Australian Sky & Telescope - May 2018

18 AUSTRALIAN SKY & TELESCOPE May | June 2018

COSMIC DAWN by James Geach

T

here’s a certain exhilaration in hunting for the

most distant galaxies in the universe. It’s that

feeling of pushing back the boundaries. Seeing a

bit farther than anyone before you.

But I bet if you ask most astrophysicists who

study the formation and evolution of galaxies, it’s not the

distance in space that excites them, but the distance in time.

Electromagnetic radiation travels at a finite speed, and the

light emitted by distant galaxies can take billions of years

— large fractions of the age of the universe — to reach us.

We see those distant galaxies as they were in the past, at the

moment the light left them. Our ultimate goal is to detect the

light emitted by the first galaxies: the cosmic dawn.

Remarkably, we’re getting close. Any long-exposure

image will reveal a universe teeming with galaxies; current

estimates put the total number of galaxies around the trillion

mark. As the sensitivity of CCD cameras improves and the

size of telescopes increases, it has become routine for deep

astronomical surveys to detect galaxies seen at a time when

the universe was a small fraction of its current age.

These infant galaxies are much too far away for light-

years to be a useful measure. Rather, we use the galaxies’

cosmological redshift as a proxy for their distance.

The universe — space itself — has been expanding ever

since it came into existence in the Big Bang. The wavelength

of an electromagnetic wave travelling through the expanding

universe will stretch so that blue light emitted by a distant

galaxy will have become red light by the time we detect

it. The amount the wavelength has stretched tells us how

much the universe has expanded since the photon began its

journey. If we know the history of the expansion (since space

hasn’t grown at a steady rate), then we can use redshift to

say something about how far away that galaxy is and how far

back in time we are seeing it.

Although we have observed galaxy formation over most of

the history of the universe, we still don’t know how galaxies

first formed. We have models and simulations, but not the

observations to test them. This is why we need to see the

earliest galaxies. In fact, what we’d really like to know is when

we stop seeing galaxies.

Cosmic dropouts

Measuring the redshift of very distant galaxies isn’t easy.

They’re faint, and typically we need to disperse the small

amount of light we do detect into a spectrum, thinning it out

like butter spread over hot toast, in order to see how much

light we receive at each wavelength. The spectrum allows us

to identify key features, such as the spike-like emission lines

of excited hydrogen gas, to determine precisely how much

those lines have redshifted.

Astronomers are travellingbackwards in time

to observe our universe’s early history.

The

FIRST