2019-01-01_Discover

FROM TOP: NASA/ESA; ROEN KELLY/DISCOVER

from the uncertainties of local

cosmic measurements. And here the

disagreements begin.

“With the Planck data, we get

a Hubble constant of 68 or 67,”

says astrophysicist Jo Dunkley of

Princeton University. The discrepancy

with the distance ladder results

became glaring enough that Dunkley

wanted to make sure the Planck

researchers weren’t fooling themselves

in any way.

The task ended up falling to one

of Dunkley’s former

graduate students,

Graeme Addison,

now a cosmologist

at Johns Hopkins

University. Addison

began exploring yet

another way to reckon

the expansion of the

universe, this time by

looking at vibrations

from the Big Bang

that left an enormous

pattern of ripples etched

into the distribution of galaxies across

the universe. That pattern is visible in

the latest large-scale surveys. The size

of the cosmic ripples, when combined

with other information about the Big

Bang, yields a measurement of the

Hubble constant.

“It’s a third opinion on what’s going

on,” Addison says. That opinion

aligns with Planck’s value of 67,

“which suggests pretty strongly that

you can’t blame this discrepancy all on

the Planck data.”

WHAT THE HECK IS GOING ON?

Compared with old-school brawlers

like Sandage, today’s researchers

are a genial and cautious bunch.

They begin, sensibly, by considering

human error: the possibility that

somebody messed up in collecting or

analyzing the data. But that’s looking

increasingly unlikely.

“If you’d asked me three years

ago, I’d have said, ‘The distance

ladder is pretty complicated, and

there’s astrophysics that needs to

be understood.’ But my opinion on

that has changed because of the

work, mainly from Riess and his

collaborators,” Addison says. “They’ve

revisited the steps of the distance

ladder, done statistical tests, and none

of that analysis has shifted the Hubble

constant anything near the amount

you need to reconcile with Planck.”

What’s most troubling — and

exciting — is that so many lines of

evidence converge on two inconsistent

answers. The persistent gap is

forcing cosmologists to consider

that both measurements might be

correct. Perhaps the universe has

a split identity: Maybe the early

universe studied by Planck, and the

late universe studied by the Hubble

telescope, really were inconsistent, due

to some undiscovered aspect of how

the universe works.

Riess ticks off a range of

possibilities, any one of which would

qualify as a major discovery. Space

itself could have a slight curvature.

There could be an unknown type of

neutrino, a ghostly type of particle

that rarely interacts with matter. Dark

matter and dark energy could have

“funny funky” properties.

Dunkley suspects that the problems

with current scientiic understanding

may go even deeper than the kinds

of relatively minor adjustments that

Riess describes. “There’s no single

extension of the standard model of

cosmology that can explain the Planck

data and the local measurements

and this vast suite of other data,”

she says. “We’ve also got big issues

to igure out, like why is the universe

accelerating, and why did it begin

expanding in the irst place? I would

not be surprised if we’ve got a major

upheaval coming.”

The current debate may seem

smaller than the one that came before,

but it could prove large enough to

deliver a whole new universe.^ D

Corey S. Powell, a contributing

editor of Discover, also writes for the

magazine’s Out There blog. Follow him

on Twitter: @coreyspowell

92 DISCOVERMAGAZINE.COM

We know the universe is

expanding (left), but the

conflicting numbers for

the rate could mean greater

mysteries, and surprises,

are in the future.

Out

here