The challenge to identify a truly explanatory and predictive scientific paradigm describing the origin, evolution and future of the Universe‏

World View - A personal take on events

Big Bang blunder bursts the multiverse bubble

by Paul Steinhardt

Nature, June 5, 2014, Vol 510, page 9.

http://www.nature.com/polopoly_fs/1.15346!/menu/main/topColumns/topLeftColumn/pdf/510009a.pdf

Premature hype over gravitational waves highlights

gaping holes in models for the origins 

and evolution of the Universe, argues Steinhardt

The inflationary paradigm is fundamentally untestable,

and hence scientifically meaningless, he says.

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When a team of cosmologists 

announced at a press conference in March 

that they had detected gravitational waves 

generated in the first instants after the Big Bang, 

the origins of the Universe were once again major news. 

The reported discovery 

created a worldwide sensation 

in the scientific community, 

the media and the public at large 

(see Nature 507, 281–283; 2014).

According to the team 

at the BICEP2 South Pole telescope, 

the detection is at the 5–7 sigma level, 

so there is less than one chance in two million 

of it being a random occurrence. 

The results were hailed as proof 

of the Big Bang inflationary theory 

and its progeny, the multiverse. 

Nobel prizes were predicted 

and scores of theoretical models spawned. 

The announcement also influenced 

decisions about academic appointments 

and the rejections of papers and grants. 

It even had a role 

in governmental planning 

of large-scale projects.

The BICEP2 team identified 

a twisty (B-mode) pattern 

in its maps of polarization 

of the cosmic microwave background, 

concluding that this was a detection 

of primordial gravitational waves. 

Now, serious flaws in the analysis 

have been revealed that transform 

the sure detection into no detection. 

The search for gravitational waves must begin anew. 

The problem is that other effects, 

including light scattering from dust 

and the synchrotron radiation 

generated by electrons moving around 

galactic magnetic fields within our own Galaxy, 

can also produce these twists.

The BICEP2 instrument detects radiation 

at only one frequency, so cannot distinguish 

the cosmic contribution from other sources. 

To do so, 

the BICEP2 team used measurements 

of galactic dust collected 

by the Wilkinson Microwave Anisotropy Probe 

and Planck satellites, 

each of which operates 

over a range of other frequencies. 

When the BICEP2 team did its analysis, 

the Planck dust map had not yet been published, 

so the team extracted data from a preliminary map 

that had been presented several months earlier. 

Now a careful reanalysis 

by scientists at Princeton University 

and the Institute for Advanced Study, 

also in Princeton, has concluded 

that the BICEP2 B-mode pattern 

could be the result mostly 

or entirely of foreground effects 

without any contribution 

from gravitational waves. 

Other dust models considered 

by the BICEP2 team 

do not change this negative conclusion, 

the Princeton team showed 

(R. Flauger, J. C. Hill and D. N. Spergel, 

preprint at http://arxiv.org/abs/1405.7351; 2014).

The sudden reversal 

should make the scientific community 

contemplate the implications for the future 

of cosmology experimentation and theory. 

The search for gravitational waves is not stymied. 

At least eight experiments, 

including BICEP3, 

the Keck Array and Planck, 

are already aiming at the same goal.

This time, the teams can be assured 

that the world will be paying close attention. 

This time, acceptance 

will require measurements 

over a range of frequencies 

to discriminate from foreground effects, 

as well as tests to rule out 

other sources of confusion. 

And this time, 

the announcements 

should be made 

after submission to journals 

and vetting by expert referees. 

If there must be a press conference, 

hopefully the scientific community 

and the media will demand 

that it is accompanied 

by a complete set of documents, 

including details of the systematic analysis 

and sufficient data to enable objective verification.

The BICEP2 incident 

has also revealed 

a truth about inflationary theory. 

The common view is that 

it is a highly predictive theory. 

If that was the case 

and the detection of gravitational waves 

was the ‘smoking gun’ proof of inflation, 

one would think that non-detection 

means that the theory fails. 

Such is the nature of normal science. 

Yet some proponents of inflation 

who celebrated the BICEP2 announcement 

already insist that the theory is equally valid 

whether or not gravitational waves are detected. 

How is this possible?

The answer given by proponents is alarming: 

the inflationary paradigm is so flexible 

that it is immune to experimental and observational tests. 

First, inflation is driven by a hypothetical scalar field, 

the inflaton, which has properties that can be adjusted 

to produce effectively any outcome. 

Second, inflation does not end 

with a universe with uniform properties, 

but almost inevitably leads to a multiverse 

with an infinite number of bubbles, 

in which the cosmic and physical properties 

vary from bubble to bubble. 

The part of the multiverse that we observe 

corresponds to a piece of just one such bubble. 

Scanning over all 

possible bubbles in the multiverse, 

everything that can physically happen 

does happen an infinite number of times. 

No experiment can rule out a theory 

that allows for all possible outcomes. 

Hence, the paradigm of inflation is unfalsifiable.

This may seem confusing 

given the hundreds of theoretical papers 

on the predictions of this or that inflationary model. 

What these papers 

typically fail to acknowledge 

is that they ignore the multiverse 

and that, even with this unjustified choice, 

there exists a spectrum of other models 

which produce all manner 

of diverse cosmological outcomes. 

Taking this into account, 

it is clear that the inflationary paradigm 

is fundamentally untestable, 

and hence scientifically meaningless.

Cosmology 

is an extraordinary science 

at an extraordinary time. 

Advances, including 

the search for gravitational waves, 

will continue to be made 

and it will be exciting to see 

what is discovered in the coming years. 

With these future results in hand, 

the challenge for theorists will be 

to identify a truly explanatory 

and predictive scientific paradigm 

describing the origin, 

evolution and future of the Universe.