2019-07-01_Australian_Sky_&_Telescope

20 AUSTRALIAN SKY & TELESCOPE July 2019

GRAVITATIONAL LENSING The light from remote galaxies

and quasars can be split into multiple images by the gravity

of a massive foreground object, such as a huge elliptical

galaxy or a galaxy cluster. Brightness changes in the lensed

object (for example, a supernova explosion in a galaxy, or

the temporary flickering of a quasar core) arrive at Earth

at different epochs, because each light path has its own

associated travel time. From the time differences — and a

precise model of the mass distribution of the foreground

lens — it is possible to calculate the distances travelled,

measuring the Hubble constant to a precision of some

3%, according to Sherry Suyu (Max Planck Institute for

Astrophysics, Germany). Her team’s latest results yield a value

for H0 between 70.2 and 74.6 km/s/Mpc, in fair agreement

with the ‘local’ Cepheid/supernova method.

BARYON ACOUSTIC OSCILLATIONS (BAOS) Some 370,000

years after the Big Bang, free electrons combined with atomic

nuclei, and the universe became transparent to radiation.

Large-scale pressure waves in the primordial gas, produced

by the earlier photon pressure, suddenly froze out. As a

result, every high-density lump of matter was surrounded by a

sphere with a higher-than-average density at a characteristic

distance of something between 400,000 and 500,000 light-

years. Consequently, the chance of two galaxies forming at

this mutual distance was higher than average. According to

Matthew Colless (Australian National University), it’s a small

effect, invisible to the eye, but statistical studies of the current

three-dimensional distribution of 1.5 million galaxies do

reveal it, albeit at a much larger characteristic scale, thanks

to the expansion of the universe. The ‘peak’ now appears at a

mutual distance of 147 megaparsecs (480 million light-years),

and from this value, a Hubble constant of 67 km/s/Mpc can

be derived, in accordance with the ‘cosmological’ method.

CLUSTERING AND WEAK LENSING Combined with

baryon acoustic oscillations and the density of baryons,

observations of the clustering properties of galaxies over

time and measurements of so-called weak lensing yield an

independent, pretty precise value of the Hubble constant.

This approach has been followed by the Dark Energy Survey

— a comprehensive survey of the positions, redshifts and

shapes of a few hundred million galaxies, carried out with

the 520-megapixel Dark Energy Camera on the 4-metre

Blanco Telescope at Cerro Tololo, Chile. Preliminary results,

published in November 2018, arrive at a value for H0 of

OTHER WAYS TO MEASURE THE HUBBLE CO

S THEWALLMeasurementsofthecosmicexpansionratesplit,

with studies that use distance scales set in the early universe

favouring lower values than those that use phenomena such as

supernovae.

H^0

MEASUREMENTS: GREGG DINDERMAN /

S&T

;^ EXPLOSION GRAPHIC: TATIANAZAETS / ISTOCK / GETTY IMAGES PLUS

COSMIC EXPANSION PUZZLE

The current expansion rate of the universe can be derived from pre asurements of

distances and ‘recession’ velocities of remote galaxies, or calculate cosmological models

and the observed properties of the cosmic microwave backg approaches are

discussed in the main story. But there are other ways to mea le constant, H0:

million light-years, with an error margin of 15%. Combining

this with the known redshift of the galaxy yields a (rather

imprecise) value for the Hubble constant of 70 km/s/

Mpc, smack in the middle of the ‘local’ and ‘cosmological’

values. However, according to Bernard Schutz (Cardiff

University, UK), future gravitational-wave detectors like the

Einstein Telescope and the Cosmic Explorer will be able

to ‘see’ every binary neutron star merger in the observable

universe, and eventually, the precision of this method may

improve to 0.1%.

67.4 km/s/Mp d agreement with the

‘cosmological

GRAVITATIO gust 2017, astronomers

and physicist of gravitational waves

— minute ripp — from a pair of colliding

neutron stars.Fromthedetect atterns, it was

possible to deduce the masses lliding stars and

the corresponding energy that tedin theformo

gravitational waves. Because the wave

as they travelled, measuring the amplit

here on Earth then provided an indepen

estimate for the host galaxy of the merger (NGC 4993): 143

BOSS

(baryonacoustic

oscillations)

DarkEnergySurvey

(cosmicstructure+

lensing+ baryondensity)

Planck

(CMB)

Expansionrate(km/s/Mpc)

60 65 70 75 80

SH0ES

(Type1a+ Cepheids)

TypeIa

Lensedquasars

Gravitationalwaves

Cepheids

Hubble Constant Measurements

Primordial imprints Phenomena