- 3769 - THEORY OF GRAVITY - is it universal? Everything in the universe has gravity and feels it. Yet this most common of all fundamental forces is also the one that presents the biggest challenges to physicists. Albert Einstein’s theory of general relativity has been remarkably successful in describing the gravity of stars and planets, but it doesn’t seem to apply perfectly on all scales.
--------------------- 3769 - THEORY OF GRAVITY - is it universal?
- Albert Einstein’s theory of general relativity has been remarkably successful in describing the gravity of stars and planets, but it doesn’t seem to apply perfectly on all scales.
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- General relativity has passed many years of observational tests, from Eddington’s measurement of the deflection of starlight by the Sun in 1919 to the recent detection of gravitational waves. However, gaps in our understanding start to appear when we try to apply it to extremely small distances, where the laws of quantum mechanics operate, or when we try to describe the entire universe. The very large and the very small open to new mysteries.
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- Quantum theory predicts that empty space, the vacuum, is packed with energy. We do not notice its presence because our devices can only measure changes in energy rather than its total amount.
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- According to Einstein‘s theories, the vacuum energy has a repulsive gravity, it pushes the empty space apart. In 1998, it was discovered that the expansion of the universe is in fact accelerating. However, the amount of vacuum energy, or dark energy as it has been called, necessary to explain the acceleration is many orders of magnitude smaller than what quantum theory predicts. That is, the math does not work?
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- Hence the big question, dubbed “the old cosmological constant problem”, is whether the vacuum energy actually gravitates, exerting a gravitational force and changing the expansion of the universe.
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- If yes, then why is its gravity so much weaker than predicted? If the vacuum does not gravitate at all, what is causing the cosmic acceleration? We don’t know what dark energy is, but we need to assume it exists in order to explain the universe’s expansion.
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- We also need to assume there is a type of invisible matter presence, dubbed dark matter, to explain how galaxies and clusters evolved to be the way we observe them today.
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- These assumptions are wthin the “standard cosmological theory“, called the “lambda cold dark matter (LCDM) model”, suggesting there is 70% dark energy, 25% dark matter and 5% ordinary matter in the cosmos. And this model has been remarkably successful in fitting all the data collected by cosmologists over the past 20 years.
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- But the fact that most of the universe is made up of dark forces and substances, taking odd values that don’t make sense, has prompted many physicists to wonder if Einstein’s theory of gravity needs modification to describe the entire universe.
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- A new twist appeared a few years ago when it became apparent that different ways of measuring the rate of cosmic expansion, dubbed the Hubble constant, give different answers, a problem known as the “Hubble tension“.
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- The disagreement, or tension, is between two values of the Hubble constant. One is the number predicted by the LCDM cosmological model, which has been developed to match the light left over from the Big Bang (the cosmic microwave background radiation). The other is the expansion rate measured by observing exploding stars known as supernovas in distant galaxies.
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- Many theoretical ideas have been proposed for ways of modifying LCDM to explain the Hubble tension. Among them are alternative gravity theories. We can design tests to check if the universe obeys the rules of Einstein’s theory. General relativity describes gravity as the curving or warping of space and time, bending the pathways along which light and matter travel. It predicts that the trajectories of light rays and matter should be bent by gravity in the same way.
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- To find out whether general relativity is correct on large scales, astronomers, for the first time, simultaneously investigated three aspects of it. These were the “expansion of the universe“, the “effects of gravity on light” and the “effects of gravity on matter“.
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- Astronomers reconstructed the gravity of the universe through cosmic history in a computer model based on these three parameters. They could estimate the parameters using the cosmic microwave background data from the Planck satellite, supernova catalogues as well as observations of the shapes and distribution of distant galaxies by the SDSS and DES telescopes. We then compared this reconstruction to the prediction of the LCDM model, essentially Einstein’s model.
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- They found interesting hints of a possible mismatch with Einstein’s prediction, although with rather low statistical significance. This means that there is nevertheless a possibility that gravity works differently on large scales, and that the theory of general relativity may need to be tweaked.
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- The study also found that it is very difficult to solve the Hubble tension problem by only changing the theory of gravity. The full solution would probably require a new ingredient in the cosmological model, present before the time when protons and electrons first combined to form hydrogen just after the Big Bang, such as a special form of dark matter, an early type of dark energy or primordial magnetic fields.
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- To offer a contrary idea 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.
They say that there is a clear, unambiguous signal in the cosmos which could eliminate inflation as a possibility known as the cosmic graviton background (CGB).
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- Inflation was theorised to explain various fine-tuning challenges of the so-called hot Big Bang model. It also explains the origin of structure in our Universe as a result of quantum fluctuations.
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- 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. Some astronomers 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 may have begun not with a bang but with a bounce from a previously contracting cosmos.
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- The 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. The actual edge of the observable universe is at the distance that any signal could have traveled at the speed-of-light limit over the 13.8 billion years that elapsed since the birth of the Universe.
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- As a result of the expansion of the universe, this edge is currently located 46.5 billion light years away. The spherical volume within this boundary is like an archaeological dig centered 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 this horizon is unknown.
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- 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. The Universe allows neutrinos to travel freely without scattering from approximately a second after the Big Bang, when the temperature was ten billion degrees. The present-day universe must be filled with relic neutrinos from that time.
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- We can go even further back, however, by tracing “gravitons“, particles which mediate the force of gravity. The Universe was transparent to gravitons all the way back to the earliest instant traced by known physics, the Planck time, 10^-43 seconds, when the temperature was the highest conceivable: 10^ 32 degrees.
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- 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).
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- 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. If the CGB were detected, clearly this would rule out cosmic inflation, which does not allow for its existence.
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- The 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.
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- To claim a definitive detection of the CGB 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, this signal may be within our reach in future.
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- Astronomy is a never ending science. But, I’ll keep working on it.
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December 3, 2022 THEORY OF GRAVITY - is it universal? 3741
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