Quanta Magazine

https://www.quantamagazine.org/the-case-for-spectral-distortions-in-the-cosmic-microwave-background-20150630/ June 30, 2015

Why the Big Bang’s Light May Have a TiltScientists haven’t rigorously tested the cosmic microwave background for a revealing shift in 25years. A new experiment aims to change that.

By Maggie McKee

Thecosmic microwave background radiation still holds secrets about the Big Bang.

Half a century ago, astronomers got their first look at the infant universe: a haze of soft light thatsuffused the entire sky. This cosmic microwave background (CMB) radiation seemed to indicate thatthe early cosmos was remarkably uniform — a hot, dense fireball that expanded and cooled over thenext 14 billion years. It was the world’s first beacon from the Big Bang.

Like a slowly developing Polaroid, our understanding of this radiation has come into focus gradually.In 1990, NASA’s Cosmic Background Explorer (COBE) satellite found that light from the CMB hadthe telltale spectrum of a system in equilibrium, known as a blackbody — exactly what was expectedif the universe began as a dense, scalding soup of particles and photons that all interacted with oneother. In addition, another instrument on COBE revealed slight hot and cold spots in the light.

Quanta Magazine

https://www.quantamagazine.org/the-case-for-spectral-distortions-in-the-cosmic-microwave-background-20150630/ June 30, 2015

Subsequent spacecraft, including NASA’s WMAP satellite and Europe’s Planck probe, havesharpened our view of the temperature variation, or anisotropy, even further. Yet measurementsof the CMB’s spectrum have hardly budged in that time. At the wavelengths it studied, COBE’smeasurement 25 years ago is “still the best, the gold standard,” said Jim Peebles, a physicist atPrinceton University.

Alan Kogut is proposing a space-basedobservatory to search for spectral distortions.

But more sensitive measurements should undoubtedly reveal small deviations from the blackbodycurve that COBE measured. That’s because anything that injected energy into the universe after itwas a few months old should have distorted this spectrum somewhat, said Alan Kogut, a physicist atNASA’s Goddard Space Flight Center in Greenbelt, Md.

“There’s a whole lot of things you can learn” from such distortions, he said.

Researchers discussed many of these prospects earlier this month at a conference at PrincetonUniversity celebrating 50 years of CMB studies. The potential revelations include details aboutobjects both ordinary, such as stars, and exotic, such as dark-matter particles, that CMB photonsmight encounter on their travels through space. Even more enticingly, spectrum measurementscould expose details about the first moments of the universe that no other technique could probe. Aspace mission called the Primordial Inflation Explorer (PIXIE), now in development, could look forthese spectral distortions.

A Light ShiftThe blackbody spectrum we measure today was created just a few months after the universe’s birth,when the number of photons generated in the early fireball had stabilized. “Anything that happensafterwards can distort the spectrum,” Kogut said.

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https://www.quantamagazine.org/the-case-for-spectral-distortions-in-the-cosmic-microwave-background-20150630/ June 30, 2015

In the early universe, dark matter particles thatannihilate or decay could alter the CMB. The demise of dark matter could produce ordinary particleslike neutrinos and electrons that change the wavelength of the CMB’s light.

For the first couple of decades after that, the universe was so dense that any process producingextra energy, such as the annihilation or decay of dark-matter particles, would affect all of the CMBphotons, creating what are known as mu (µ) distortions in the blackbody spectrum. In this situation,an energetic electron produced by the demise of a dark-matter particle could transfer some of itsenergy to a CMB photon, “distorting the microwave background away from a blackbody,” Kogutsaid.

Even earlier spectral distortions might also have arisen from inflation, a short but spectacularly fastperiod of expansion that many researchers think occurred in the universe’s very first moments.

According to this theory, quantum fluctuations created dimples in space-time that inflation thenamplified. Matter and radiation fell into these valleys, which eventually evolved into the firstgalaxies; the valleys explain how the chunky stew of a universe we see today emerged out of itsbrothlike past.

The valleys should all be of different widths, depending on when the fluctuations arose and how longthey had to inflate. But leading models of inflation predict that they should all have about the samedepth, since the energy scale of the inflationary field, which produced the quantum blips, is thoughtto have changed only slowly over time.

The matter and radiation that slid into the valleys rebounded, sloshing out and then oversurrounding hills into other valleys, producing the hot and cold spots in the CMB. If no energy waslost during the sloshing, these spots would vary from the average CMB temperature by about thesame amount. But some energy was lost. As the sloshing proceeded, more and more photonssplashed out of the valleys. Because of this, the smallest ones, produced toward the end of inflation,no longer appear hot or cold. The effect, known as Silk damping, erases information about the depthof smaller valleys — and the energy scale of inflation at later times — in maps of the CMB’stemperature.

Quanta Magazine

https://www.quantamagazine.org/the-case-for-spectral-distortions-in-the-cosmic-microwave-background-20150630/ June 30, 2015

Subtleshifts in the CMB’s spectrum would reveal important details about the early universe.

The energy lost during the sloshing didn’t disappear, however. It went into “heating up the universea little bit,” Kogut said. That would have shifted the CMB’s spectrum away from a blackbody. “Itbasically makes the universe look a little bit bluer — a little brighter at shorter wavelengths andcolder at longer wavelengths,” Kogut said.

Locating these spectral distortions could therefore reveal details about inflation on smaller scales,and later times, than is now possible. “That is information you could not get any other way,” saidSimon White, director of the Max Planck Institute for Astrophysics in Garching, Germany.Measurements of the depth of the dimples at later times could test how quickly the energy scale ofinflation changed, which would test competing models of the theory, Kogut said.

That’s important, said John Mather, an astrophysicist at NASA Goddard who won a Nobel Prize in2006 for measuring the CMB’s blackbody spectrum with COBE. One of the selling points of inflationis that it seems to explain the CMB’s incredible evenness across the sky — patches of sky that are farapart would have been touching before the exponential expansion of inflation. But the CMB’suniformity was discovered years before the theory of inflation was developed in the 1980s, andMather said the theory would gain credibility if it made predictions that were only later found to betrue. “It’s not all that powerful to predict something you already know,” he said.

Spectral measurements could also provide insight into the evolution of the universe at later times,when the universe had expanded enough that any injections of energy would be felt by only afraction of CMB photons, producing what are called y distortions in the spectrum.

Energy from stars, galaxies and galaxy clusters should stimulate the CMB, which could help pindown the rate at which stars form and explode and at which galaxies grow and evolve, Peebles said.“There are lots of ideas about how cosmic evolution proceeded, but not much evidence,” he said.Spectral distortion measurements would offer “a rigorous constraint on what is otherwise a veryslippery business.”

Quanta Magazine

https://www.quantamagazine.org/the-case-for-spectral-distortions-in-the-cosmic-microwave-background-20150630/ June 30, 2015

The $200 million PIXIE mission, which Kogut and his team are proposing to NASA for a potentiallaunch in 2022, could search for all of these spectral distortions. With a sensitivity about 1,000 timesas great as COBE, it could potentially study inflation’s dimples at a scale one-ten-thousandth ofwhat’s possible with the CMB hot and cold spots. The mission would also search for the signature ofgravitational waves from the early universe with a precision 100 times better than that of currentexperiments.

Despite its sensitivity, interpreting PIXIE’s results, should it launch, would be tricky. “Differentprocesses … can lead to similar distortions,” said Jens Chluba, an astrophysicist at the Instituteof Astronomy in Cambridge, England. “However, with precise measurements, different scenarios canin principle be distinguished.”

Kogut agrees. “The main source of confusion would be from dust in our own galaxy,” he said.However, he thinks that PIXIE can rigorously account for the influence of dust through itsmeasurements of the sky at 400 different wavelength bands, since dust shines more brightly atcertain colors than others.

“The low-hanging fruit has been gathered,” Peebles said of CMB measurements to date. Trying todetect the CMB’s deviations from a blackbody spectrum is “a very difficult measurement, but onethat can be done and would teach us a lot about cosmic evolution.”

Half a century after the CMB was discovered, what can we expect in the future? Peebles is sure ofone thing. “The next 50 years will be interesting.”

This article was reprinted on Wired.com.

Update July 9, 2015: Jens Chluba’s affiliation has been updated.