2019-01-01_Discover

90 DISCOVERMAGAZINE.COM

ECHOES OF EXPANSION

This all sounds great until you talk

to the equally persuasive scientists

who work with data from the Planck

satellite. From 2009 to 2013, Planck

made exacting observations of the

cosmic microwave background, the

relic radiation from the Big Bang.

Whereas the distance-ladder folks

begin nearby, Planck researchers go

deep, studying radiation from the

very edge of the visible universe and

examining it for patterns that reveal

the expansion history of the universe.

It’s a cunning approach, liberated

objects to calibrate distances to things

farther away, and so on, like climbing

the rungs of a ladder. Once you know

true distances to cosmic objects, you

can combine that information with

measurements of their motion to derive

the expansion rate of the universe.

The distance ladder is how Edwin

Hubble discovered the universe was

expanding at all, back in 1929, but using

it to deduce the rate of the expansion

proved problematic. The huge challenge

was calibrating the irst rung on the

ladder, to a type of star called a Cepheid

variable. These pulsate in a rhythm that

depends on their intrinsic luminosity.

If you measure a Cepheid’s period of

pulsations, you’ll know how bright the

star really is. Compare that with how

bright it appears, you can igure out

how far it is — and away you go.

Unfortunately, that Cepheid trick

gives only relative distances. To get

actual distances, you need to know

the exact locations of a few nearby

Cepheid stars to use as references

for all the rest. “This is what plagued

people 20 or 25 years ago. They had

so much dificulty calibrating anything

in an absolute sense,” Riess says. He

has spent much of his career trying to

rid cosmology of that plague.

In Sandage’s day, the Hubble

constant was uncertain by about

50 percent. By 2001, data from Hubble

pushed the error down to 10 percent.

In his most recent paper, Riess claims

an accuracy of 2.2 percent — “and it’s

possible to push closer to 1 percent.

That’s my goal over the next few

years, and that’s looking

feasible,” he says. As a

result, he’s conident

when he says the

value of the Hubble

constant is 73.

FROM TOP: ESA AND THE PLANCK COLLABORATION; NASA, ESA, A. FEILD (STS

CI), AND A. RIESS (STS

CI/JHU)

The parallax effect changes the apparent position of an object depending on the point of observation — it’s why your

fingers can jump around if you look at them with just your left eye, then your right eye. By measuring exactly how much

stars jump between Earth’s position in June and its position in December, astronomers can deduce their rough distances.

Planck’s early

universe map

Out

here