Australian Sky & Telescope - May 2018

Cosmological

Redshift

Redshift compares the

observed wavelength of light

with its wavelength when it was

emitted. Hydrogen, for example,

can emit light at 121.6 nm, but

in a faraway galaxy we observe

that line at redder wavelengths

due to Doppler shift.

By comparing the two

wavelengths — emitted and

observed — cosmological

redshift tells us the size of the

universe today relative to the

size of the universe then.

Here’s the equation that

describes this:

whereλis the wavelength.

Galaxies very near the Milky

Way have a redshift of nearly

0, which means that the

wavelengths we observe are

about the same as those

originally emitted. (Gravitational

attraction also drives relative

motions between galaxies,

but any resulting shifts in

wavelength are unrelated to the

universe’s expansion and hence

to the galaxies’ distances.)

The wavelengths of light from a

galaxy with a redshift of 1 (that

is, emitted in a universe roughly

half its current age) will double

by the time they reach us. A

galaxy in the early universe, at a

redshift of 10, will have had its

light stretched by a factor of 11,

pushing ultraviolet wavelengths

to much longer (redder)

wavelengths, near the infrared

part of the spectrum.

Lyman

break

(91.2 nm)

Galaxy at z=0

Wavelength

FUV

NUV B

Far-

Ultraviolet

(FUV)

Near-

Ultraviolet

(NUV)

Near-

Infrared

(I)

Blue

(B)

Visible

(V)

V I

Brightness

Lyman

break

(547.2 nm)

Galaxy atz=5

Wavelength

FUV

NUV B

Far-

Ultraviolet

(FUV)

Near-

Ultraviolet

(NUV)

Near-

Infrared

(I)

Blue

(B)

Visible

(V)

V

I

Brightness

COSMIC DROPOUTS Splitting light from a typical galaxy into its

component colours results in a spectrum like the one shown at top,

spanning far-ultraviolet to near-infrared wavelengths. Neutral hydrogen

gas absorbs light emitted below the so-called Lyman break at 91.2

nanometres. Thanks to the break, examining the light at different ilters

reveals the distance to faint, faraway galaxies without actually having

to measure their spectra. Because the expansion of the universe

stretches the wavelengths of light travelling through it, the wavelength

at which the Lyman break is observed shifts redward, so that the

galaxy becomes unobservable at shorter wavelengths.

RED SHIFT: LEAH TISCIONE /

S&T

/ J. GEACH; RED GRADIENT: HARDIK PETHANI / GETTY IMAGES

z = – 1

λobserved

λemitted

http://www.skyandtelescope.com.au 21