Cosmologists find way to verify if the universe is hotter at one end than the other

A remnant of the Big Bang, the cosmic microwave background
appears to harbor a gradient across the universe, a feature
that has puzzled cosmologists for decades. Credit: Matthew

Scientists have long observed an apparent gradient in the
cosmic microwave background but have been unable to determine
how much is real and how much is perceived. USC Dornsife
researchers appear to have found a way to an answer.

Observed from Earth, the universe appears a bit hotter at one
end than the other, at least in terms of the (CMB). But the
question plaguing cosmologists is whether that imbalance in the
CMB is real or a result of the Doppler effect.

USC Dornsife scientists Siavash Yasini and Elena Pierpaoli may
have found a way to nail down an answer.

Made most famous perhaps by Edwin Hubble, who used it to show
that the universe is expanding, the Doppler effect is the
apparent shift in the frequency of electromagnetic waves due to
the motion of bodies traveling swiftly through space. Waves
such as electromagnetic radiation—light waves, X-rays,
microwaves, etc.—appear to shift in energy, with those moving
toward an observer appearing to be higher in energy, or hotter,
than they really are. The opposite is true for waves moving
away from the observer, which appear colder.

Scientists looking at the sky see space trailing behind Earth
appearing colder than space up ahead, but it’s not clear if
that’s only the Doppler effect or an observation of a true
difference in CMB temperature. It’s a puzzle that has persisted
for decades.

Because the CMB is leftover energy from the Big Bang—when the
entire universe exploded outward from a single
point—cosmologists have assumed it is dispersed evenly. The
appearance of two poles in the universe, one warmer than the
other, must therefore be a result of the Doppler effect, a
result of the solar system careening through space.

“We think that one side of the CMB only looks hotter because we
are moving towards it, and the opposite side looks colder
because we are moving away from it,” said Yasini, a Ph.D.
student in physics and astronomy.

Astrophysicists measuring the solar system’s velocity relative
to the CMB adjust their calculations based on this assumption,
as do cosmologists studying the Big Bang and conditions shortly

But this might be a mistake after all.

“If there is an intrinsic dipole in the CMB—that is, if one
side of the sky is actually partially hotter than the opposite
side—the velocity we assign the solar system with respect to
the CMB would be incorrect,” Yasini said. This would affect how
scientists measure the speed of distant objects such as
galaxies, and theories about what happened moments after the
Big Bang could be shaken.

Running calculations for a different but related study, Yasini
and Professor of Physics and Astronomy Pierpaoli, who is
Yasini’s graduate school mentor, found an interesting detail:
The frequency spectrum of the CMB averaged across the sky will
differ if the dipole is real and not just a result of the
Doppler effect.

In other words, if the CMB is, in fact, hotter at one end of
the universe than the other, the average temperature measured
across the whole sky will be slightly different than if the CMB
is actually uniform.

Yasini and Pierpaoli’s findings will allow cosmologists
undertaking the next generation of CMB surveys to determine the
nature of the CMB dipole for the first time ever, solving the

“Now that we have a mathematical basis for finding an answer,
it just remains to make the observations,” Pierpaoli said.

If it turns out a portion of the dipole is real and not just a
result of the Doppler , astrophysicists and
astronomers will have to recalibrate all of their measurements
to get a more accurate view of the observable universe.

Just as important, cosmologists studying the Big Bang and
conditions in the very early universe will have new directions
to explore to understand how and why the CMB is dispersed
unevenly, and how the came to be as it now is.

Explore further:

What is the cosmic microwave background radiation?

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