̽��ֱ�� of Cambridge - Sunny Vagnozzi

Can cosmic inflation be ruled out?

sc604 — Thu, 03 Nov 2022 11:59:51 +0000

̽��ֱ��astrophysicists, from the ̽��ֱ�� of Cambridge, the ̽��ֱ�� of Trento, and Harvard ̽��ֱ��, say that there is a clear, unambiguous signal in the cosmos that could eliminate inflation as a possibility. Their paper, published in ̽��ֱ��Astrophysical Journal Letters, argues that this signal – known as the cosmic graviton background (CGB) – can feasibly be detected, although it will be a massive technical and scientific challenge.

“Inflation was theorised to explain various fine-tuning challenges of the so-called hot Big Bang model,” said the paper’s first author Dr Sunny Vagnozzi, from Cambridge’s Kavli Institute for Cosmology, and who is now based at the ̽��ֱ�� of Trento. “It also explains the origin of structure in our Universe as a result of quantum fluctuations.

“However, the large flexibility displayed by possible models for cosmic inflation which span an unlimited landscape of cosmological outcomes raises concerns that cosmic inflation is not falsifiable, even if individual inflationary models can be ruled out. Is it possible in principle to test cosmic inflation in a model-independent way?”

Some scientists raised concerns about cosmic inflation in 2013, when the Planck satellite released its first measurements of the cosmic microwave background (CMB), the universe's oldest light.

“When the results from the Planck satellite were announced, they were held up as a confirmation of cosmic inflation,” said Professor Avi Loeb from Harvard ̽��ֱ��, Vagnozzi’s co-author on the current paper. “However, some of us argued that the results might be showing just the opposite.”

Along with Anna Ijjas and Paul Steinhardt, Loeb was one of those who argued that results from Planck showed that inflation posed more puzzles than it solved, and that it was time to consider new ideas about the beginnings of the universe, which, for instance, may have begun not with a bang but with a bounce from a previously contracting cosmos.

̽��ֱ��maps of the CMB released by Planck represent the earliest time in the universe we can ‘see’, 100 million years before the first stars formed. We cannot see farther.

“ ̽��ֱ��actual edge of the observable universe is at the distance that any signal could have travelled at the speed-of-light limit over the 13.8 billion years that elapsed since the birth of the Universe,” said Loeb. “As a result of the expansion of the universe, this edge is currently located 46.5 billion light years away. ̽��ֱ��spherical volume within this boundary is like an archaeological dig centred on us: the deeper we probe into it, the earlier is the layer of cosmic history that we uncover, all the way back to the Big Bang which represents our ultimate horizon. What lies beyond the horizon is unknown.”

In could be possible to dig even further into the universe’s beginnings by studying near-weightless particles known as neutrinos, which are the most abundant particles that have mass in the universe. ̽��ֱ��Universe allows neutrinos to travel freely without scattering from approximately a second after the Big Bang, when the temperature was ten billion degrees. “ ̽��ֱ��present-day universe must be filled with relic neutrinos from that time,” said Vagnozzi.

Vagnozzi and Loeb say we can go even further back, however, by tracing gravitons, particles that mediate the force of gravity.

“ ̽��ֱ��Universe was transparent to gravitons all the way back to the earliest instant traced by known physics, the Planck time: 10 to the power of -43 seconds, when the temperature was the highest conceivable: 10 to the power of 32 degrees,” said Loeb. “A proper understanding of what came before that requires a predictive theory of quantum gravity, which we do not possess.”

Vagnozzi and Loeb say that once the Universe allowed gravitons to travel freely without scattering, a relic background of thermal gravitational radiation with a temperature of slightly less than one degree above absolute zero should have been generated: the cosmic graviton background (CGB).

However, the Big Bang theory does not allow for the existence of the CGB, as it suggests that the exponential inflation of the newborn universe diluted relics such as the CGB to a point that they are undetectable. This can be turned into a test: if the CGB were detected, clearly this would rule out cosmic inflation, which does not allow for its existence.

Vagnozzi and Loeb argue that such a test is possible, and the CGB could in principle be detected in future. ̽��ֱ��CGB adds to the cosmic radiation budget, which otherwise includes microwave and neutrino backgrounds. It therefore affects the cosmic expansion rate of the early Universe at a level that is detectable by next-generation cosmological probes, which could provide the first indirect detection of the CGB.

However, to claim a definitive detection of the CGB, the ‘smoking gun’ would be the detection of a background of high-frequency gravitational waves peaking at frequencies around 100 GHz. This would be very hard to detect, and would require tremendous technological advances in gyrotron and superconducting magnets technology. Nevertheless, say the researchers, this signal may be within our reach in future.

Reference:
Sunny Vagnozzi and Abraham Loeb. ‘ ̽��ֱ��Challenge of Ruling Out Inflation via the Primordial Graviton Background.’ ̽��ֱ��Astrophysical Journal Letters (2022). DOI: 10.3847/2041-8213/ac9b0e

Adapted in part from a piece on Medium by Avi Loeb.

Astrophysicists say that cosmic inflation – a point in the Universe’s infancy when space-time expanded exponentially, and what physicists really refer to when they talk about the ‘Big Bang’ – can in principle be ruled out in an assumption-free way.

Is it possible in principle to test cosmic inflation in a model-independent way?

Sunny Vagnozzi

A Ijjas, PJ Steinhardt and A Loeb (Scientific American, February 2017)

Cosmic inflation is a popular scenario for the earliest phase in the evolution of the Universe

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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Have we detected dark energy? Cambridge scientists say it’s a possibility

sc604 — Wed, 15 Sep 2021 15:36:10 +0000

A new study, led by researchers at the ̽��ֱ�� of Cambridge and reported in the journal Physical Review D, suggests that some unexplained results from the XENON1T experiment in Italy may have been caused by dark energy, and not the dark matter the experiment was designed to detect.

They constructed a physical model to help explain the results, which may have originated from dark energy particles produced in a region of the Sun with strong magnetic fields, although future experiments will be required to confirm this explanation. ̽��ֱ��researchers say their study could be an important step toward the direct detection of dark energy.

Everything our eyes can see in the skies and in our everyday world – from tiny moons to massive galaxies, from ants to blue whales – makes up less than five percent of the universe. ̽��ֱ��rest is dark. About 27% is dark matter – the invisible force holding galaxies and the cosmic web together – while 68% is dark energy, which causes the universe to expand at an accelerated rate.

“Despite both components being invisible, we know a lot more about dark matter, since its existence was suggested as early as the 1920s, while dark energy wasn’t discovered until 1998,” said Dr Sunny Vagnozzi from Cambridge’s Kavli Institute for Cosmology, the paper’s first author. “Large-scale experiments like XENON1T have been designed to directly detect dark matter, by searching for signs of dark matter ‘hitting’ ordinary matter, but dark energy is even more elusive.”

To detect dark energy, scientists generally look for gravitational interactions: the way gravity pulls objects around. And on the largest scales, the gravitational effect of dark energy is repulsive, pulling things away from each other and making the universe’s expansion accelerate.

About a year ago, the XENON1T experiment reported an unexpected signal, or excess, over the expected background. “These sorts of excesses are often flukes, but once in a while they can also lead to fundamental discoveries,” said co-author Dr Luca Visinelli, from Frascati National Laboratories in Italy. “We explored a model in which this signal could be attributable to dark energy, rather than the dark matter the experiment was originally devised to detect.”

At the time, the most popular explanation for the excess were axions – hypothetical, extremely light particles – produced in the Sun. However, this explanation does not stand up to observations, since the amount of axions that would be required to explain the XENON1T signal would drastically alter the evolution of stars much heavier than the Sun, in conflict with what we observe.

We are far from fully understanding what dark energy is, but most physical models for dark energy would lead to the existence of a so-called fifth force. There are four fundamental forces in the universe, and anything that can’t be explained by one of these forces is sometimes referred to as the result of an unknown fifth force.

However, we know that Einstein’s theory of gravity works extremely well in the local universe. Therefore, any fifth force associated to dark energy is unwanted and must be hidden, or screened, when it comes to small scales, and can only operate on the largest scales where Einstein's theory of gravity fails to explain the acceleration of the Universe. To hide the fifth force, many models for dark energy are equipped with so-called screening mechanisms, which dynamically hide the fifth force.

Vagnozzi and his co-authors constructed a physical model, which used a type of screening mechanism known as chameleon screening, to show that dark energy particles produced in the Sun’s strong magnetic fields could explain the XENON1T excess.

“Our chameleon screening shuts down the production of dark energy particles in very dense objects, avoiding the problems faced by solar axions,” said Vagnozzi. “It also allows us to decouple what happens in the local very dense Universe from what happens on the largest scales, where the density is extremely low.”

̽��ֱ��researchers used their model to show what would happen in the detector if the dark energy was produced in a region of the Sun called the tachocline, where the magnetic fields are particularly strong.

“It was really surprising that this excess could in principle have been caused by dark energy rather than dark matter,” said Vagnozzi. “When things click together like that, it’s really special.”

Their calculations suggest that experiments like XENON1T, which are designed to detect dark matter, could also be used to detect dark energy. However, the original excess still needs to be convincingly confirmed. “We first need to know that this wasn’t simply a fluke,” said Visinelli. “If XENON1T actually saw something, you’d expect to see a similar excess again in future experiments, but this time with a much stronger signal.”

If the excess was the result of dark energy, upcoming upgrades to the XENON1T experiment, as well as experiments pursuing similar goals such as LUX-Zeplin and PandaX-xT, mean that it could be possible to directly detect dark energy within the next decade.

Reference:
Sunny Vagnozzi et al. ‘Direct detection of dark energy: the XENON1T excess and future prospects.’ Physical Review D (2021). DOI: 10.1103/PhysRevD.104.063023

Dark energy, the mysterious force that causes the universe to accelerate, may have been responsible for unexpected results from the XENON1T experiment, deep below Italy’s Apennine Mountains.

It was surprising that this excess could in principle have been caused by dark energy rather than dark matter. When things click together like that, it’s really special.

Sunny Vagnozzi

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