- 2843 - BIG BANG - a theory in crisis? A series of powerful observations has made it clear that our universe has expanded for billions of years, emerging from the hot, dense state we call the Big Bang. When we compare the results from different kinds of measurements, the expansion rate of the universe, the temperature patterns in the light released when the first atoms formed, the abundances of various chemical elements, and the distribution of galaxies and other large-scale structures, we find stunning agreement of how the physics all happened. Why it happened is still a mystery.
--------------------------- 2843 - BIG BANG - a theory in crisis?
- Each of the above lines of evidence supports the conclusion that our universe expanded and evolved in just the way that the Big Bang theory predicts. From this perspective, our universe appears to be remarkably “comprehensible“. But, cosmologists have struggled, if not outright failed, to understand essential facets of the universe. The theory needs improvement.
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- We know almost nothing about “dark matter” and “dark energy“, which together make up more than 95 percent of the total mass/energy in existence today. We don’t understand how the universe’s protons, electrons, and neutrons could have survived the aftereffects of the Big Bang. It started from “nothing” it should have all collapsed back in to “nothing“, but, it didn’t.
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- In fact, everything we know about the laws of physics tells us that these matter particles should have been destroyed by antimatter long ago. And in order to make sense of the universe as we observe it, cosmologists have been forced to conclude that space, during its earliest moments, must have undergone a brief and spectacular period of hyperfast expansion, an event known as “cosmic inflation“. Yet we know next to nothing about this era of cosmic history.
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- How did we discover dark matter? The Coma Cluster packs thousands of galaxies into a sphere measuring more than 20 million light-years across. Fritz Zwicky discovered dark matter in this cluster in the 1930s when he deduced that the galaxies are moving too fast to stay together unless the cluster contains nearly 10 times as much matter as what can be seen. Invisible mater therefore dark matter.
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- With the goal of identifying the individual particles that make up this dark matter, scientists have designed and built a series of impressive experiments. Yet, to date, no such “particles” have appeared.
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- Even powerful particle accelerators like the Large Hadron Collider have revealed nothing that moves us closer to resolving any of these cosmic mysteries. And despite having measured the expansion history and large-scale structure of the universe in ever increasing detail, we have not gained any substantively greater understanding of the nature of “dark energy“, the force that seems to be accelerating the expansion of the cosmos.
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- What have we learned by “not discovering” dark matter? Dark matter is likely the most celebrated problem facing modern cosmologists. Astronomers have determined that most of the matter in our universe does not consist of atoms or any other known substances, but of something else, something that does not appreciably radiate, reflect, or absorb light. It does not get any darker than that.
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- Despite not knowing much about the nature of dark matter, cosmologists often speculate about the kinds of particles that might make up this substance. In particular, researchers have long recognized that if dark matter particles interact through a force that is approximately as powerful as the weak nuclear force (which governs radioactive decay), then the number of these particles that should have emerged from the Big Bang would roughly match the measured abundance of dark matter found in the universe today.
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- With this in mind, weakly interacting massive particles, WIMPs, became the best guess for dark matter’s nature. WIMPs that scientists thought they knew how to detect the particles and study their properties. Motivated by this goal, physicists engaged in an ambitious experimental program to identify these WIMPs and learn how they were forged in the Big Bang.
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- Over the past couple of decades, researchers have deployed a succession of increasingly sensitive dark matter detectors in deep-underground laboratories that are capable of detecting individual collisions between a dark matter particle and the atoms that make up the target.
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- These sophisticated experiments performed perfectly, yet no such collisions have been observed. A decade ago, many scientists were optimistic that these experiments would bear fruit. But dark matter has turned out to be very different, and far more elusive, than scientists had once imagined.
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- Although it’s still possible that dark matter could consist of some form of difficult-to-detect WIMPs, the lack of any signal from underground experiments has led many physicists to shift their focus toward other dark matter candidates.
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- One such contender is a hypothetical ultralight particle known as an “axion“. Axions are predicted according to a theory proposed by particle physicists in 1977. Although scientists are searching for axions in experiments that use powerful magnetic fields to convert them into photons, these investigations have yet to place very strict constraints on the properties of these particles.
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- Another possibility that could explain why dark matter has been so difficult to detect is that the first moments of the universe may have played out much differently than cosmologists have long imagined.
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- Take the case of the conventional WIMP. Calculations show that the fledgling universe should have produced vast quantities of these particles during the first millionth of a second after the Big Bang, when they reached a state of equilibrium with the surrounding plasma of quarks, gluons, and other subatomic particles.
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- The number of WIMPs that could have survived these conditions and ultimately contributed to the dark matter found throughout today’s universe depends on how, and how often, they interacted.
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- But, when carrying out calculations such as these, scientists generally assume that space expanded steadily during the first fraction of a second, without any unexpected events or transitions. It is entirely plausible that this simply was not the case.
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- Although cosmologists know a great deal about how our universe expanded and evolved over most of its history, they know relatively little about the first seconds that followed the Big Bang and next to nothing about the first trillionth of a second.
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- When it comes to how our universe may have evolved, or to the events that may have taken place during these earliest moments, we have essentially no direct observations on which to rely. This era is hidden from view, buried beneath impenetrable layers of energy, distance, and time.
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- Our understanding of this period of cosmic history is, in many respects, little more than an informed guess based on inference and extrapolation. Look far enough back in time, and almost everything we know about our universe could have been different. Matter and energy existed in different forms than they do today, and they may have experienced forces that have not yet been discovered.
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- Key events and transitions may have taken place that science has yet to develop. Matter likely interacted in ways that it no longer does, and space and time themselves may have behaved differently than they do in the world we know.
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- With this in mind, many cosmologists have begun to consider the possibility that our failure to detect the particles that make up dark matter might be telling us not only about the nature of dark matter itself, but also about the era in which it was created. By studying dark matter, scientists are learning about the first moments after the Big Bang.
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- How fast is space expanding? In 1929, Edwin Hubble discovered that galaxies are moving away from us at speeds proportional to their distances. This provided the first clear evidence that our universe is expanding. Ever since, the current rate of this expansion, called the Hubble constant, has been one of the key properties of our universe that cosmologists study.
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- As recently as the 1990s, textbooks often quoted values ranging from as low as 50 to as high as 100 kilometers per second for every million parsecs separating two points in space, usually written as 50 to 100 kilometers / second / Mpc. (One megaparsec [Mpc] equals 3.26 million light-years.)
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- Although the precision of these measurements has improved considerably over the past two decades, no consensus yet exists regarding the correct value for this quantity. In fact, as these measurements have improved, the results from different methods seem to disagree with one another even more.
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- One way to determine the Hubble constant is to directly measure how fast objects are moving away from us, just as Hubble did in 1929. For his measurements, Hubble used a special class of pulsating stars known as Cepheid variables, whose intrinsic luminosities track nicely with the periods over which they brighten and fade. For example: Once you know it is a 100 watt bulb you can measure the intensity and calculate how far the light bulb is away from you.
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- Modern cosmologists continue to use Cepheids for this purpose, but they also employ other classes of objects, including type 1a supernovae, exploding white dwarfs that all have the same approximate luminosity. When researchers combine the latest data, they find that the universe is currently expanding at a rate of about 72 to 76 km/s/Mpc.
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- Cosmologists also can infer the value of the Hubble constant by studying the primordial light released when the first atoms formed some 380,000 years after the Big Bang. The detailed temperature patterns of this light, known as the “cosmic microwave background” serve as a map that shows how matter was distributed throughout the universe at that time.
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- When scrutinized, this map reveals many details about our young universe, including how much matter and other forms of energy were present, as well as how fast space was expanding. It also tells us that the Hubble constant is about 67 km/s/Mpc, a significantly smaller value than cosmologists have found through more direct measurements.
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- What does this measurement mismatch mean for our universe? Assuming that these studies have correctly accounted for all the systematic uncertainties inherent in the observations, these two ways of determining the Hubble constant appear to be incompatible, at least within the context of the standard cosmological model.
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- To make these discrepant results mutually consistent, astronomers would be forced to change how we think the cosmos expanded and evolved, or to reconsider the forms of matter and energy in the universe during the first few hundred thousand years following the Big Bang.
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- According to Einstein’s general theory of relativity, the rate at which space expands depends on the density of matter and other forms of energy it contains. When cosmologists infer the value of the Hubble constant from the cosmic microwave background, they have to make assumptions about the amounts of dark matter, neutrinos, and other substances that were present.
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- Perhaps the simplest way to explain the tension between the different measurements of the Hubble constant would be to hypothesize that the cosmos contained more energy than expected during the first hundred thousand years or so following the Big Bang.
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- This energy might have taken the form of an exotic species of light and feebly interacting particles, or of some kind of dark energy associated with the vacuum of space itself that has long since disappeared from the universe. Or, perhaps, there is something else we don’t understand about this era of cosmic history. We simply do not yet know how to resolve this intriguing mystery.
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- Despite decades of effort, the nature of dark matter remains unknown, and the problem of dark energy seems nearly intractable. I have written dozens of reviews about dark energy and dark matter. ( Index of these reviews is available.)
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- We simply do not know how the particles that make up the atoms in our universe managed to survive the first moments of the Big Bang, and we still know little about cosmic inflation, how it played out, or how it came to an end, assuming that something like inflation happened at all.
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- The European Space Agency’s Planck satellite has captured the best data on the cosmic microwave background radiation. Combining these results with the standard model describing the universe produces a Hubble constant that is slightly but unequivocally smaller than that gleaned from nearby galaxies.
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- The physicists of 1904 had not yet been able to address a few challenges. The medium through which they believed light traveled, the luminiferous ether, should have induced variations in the speed of light, and yet light always moves through space at the same rate.
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- Astronomers observed the orbit of Mercury to be slightly different from what Newtonian physics predicted, leading some to suggest that an unknown planet, dubbed “Vulcan“, might be perturbing Mercury’s trajectory.
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- Physicists in 1904 had no idea what powered the Sun, no known chemical or mechanical process could possibly generate so much energy over such a long time. Scientists knew various chemical elements emitted and absorbed light with specific patterns, none of which physicists had the slightest idea how to explain. In other words, the inner workings of the atom remained a total and utter mystery.
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- Although few saw it coming, in hindsight, it’s clear that these problems were heralds of a revolution in physics. And in 1905, the revolution arrived, ushered in by a young Albert Einstein and his new “theory of relativity“.
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- We now know that the luminiferous ether does not exist and that there is no planet Vulcan. Instead, these fictions were symptoms of the underlying failure of Newtonian physics. Relativity beautifully solved and explained each of these mysteries without any need for new substances or planets.
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- Furthermore, when scientists combined relativity with the new theory of quantum physics, it became possible to explain the Sun’s longevity, as well as the inner workings of atoms. These new theories even opened doors to new and previously unimagined lines of inquiry, including that of cosmology itself.
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- Scientific revolutions can profoundly transform how we see and understand our world. But radical change is never easy to see coming. There is probably no way to tell whether the mysteries faced by cosmologists today are the signs of an imminent scientific revolution or merely the last few loose ends of an incredibly successful scientific endeavor.
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- There is no question that we have made incredible progress in understanding our universe, its history, and its origin. But it is also undeniable that we are profoundly puzzled, especially when it comes to the earliest moments of cosmic history.
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- The “muon anomally” may seem small, but this discrepancy too is one of the longest-standing anomalies in particle physics. Here, “anomaly” means a statistically significant experimental divergence from theoretical prediction. The theory in this case is the standard model of particle physics.
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- The muon anomaly is not alone in contesting the standard model: other anomalies concern bottom quarks, neutrinos, and kaons. In recent years, these anomalies have taken on a new level of importance as possible routes to “new physics,” an umbrella term for phenomena unexplained by the “standard model“.
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- There are more conspicuous paths to new physics, such as discovering a dark matter particle or unifying gravity with quantum physics. But these big problems have remained stubbornly out of reach, so many particle physicists are looking for inspiration from smaller problems.
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- Anomalies force experimentalists to find ways of reducing systematic uncertainties, analyzing their data, and conducting more precise searches. Theorists must grapple with anomalous results by building models that often assume new particles, such as a cousin to the Z boson, a relative to the Higgs boson, or even a hybrid between the two types of matter particles, leptons and quarks.
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- With more data coming in from experiments like “Muon g-2’ and “Belle II“, researchers are waiting to see if any anomalies become discoveries. Over the past few decades, just about every experimental result in conflict with the standard model’s predictions has disappeared over time, becoming less and less of a discrepancy as more data are taken.
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- Anomalies are discrepancies significant to 3 sigma, which corresponds to a fluke that would occur in one-in-740 runs of the experiment. Results below 3 sigma are frequently overturned.
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- Most anomalies never make it to 5 sigma (equivalent to a one-in-3.5-million-chance fluke), which is required to claim a discovery. But even when they do, it’s still not time to break out the champagne.
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- In 2011, the “OPERA experiment” infamously reported seeing neutrinos moving faster than the speed of light. Their observation was significant to more than 6 sigma but turned out to be an experimental error.
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- A few years later, an excess of two-photon events at the Large Hadron Collider at CERN seemed to suggest the existence of a new particle with a mass of 750 GeV. Theorists conjectured that such a particle could be evidence of supersymmetry, a mathematically elegant theory for new physics that many have long hoped to be true.
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- The excitement led to over 500 papers being published on this anomaly. But eventually, it too disappeared. There was no experimental error, but the 3.5-sigma result faded as further data showed that it was no more than a statistical fluctuation.
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- Anomalies are as much the product of theoretical predictions as they are of experimental measurements. The experimental value for the muon’s magnetic moment has remained essentially unchanged since 2001, when it was last measured. But the anomaly has gotten stronger, now reaching 3.3 sigma, as theoretical calculations of the muon’s magnetic interactions have improved.
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- Not every discrepancy is seen as an anomaly. Last year, two groups made separate estimates of a parameter governing the decay of the “lambda hyperon”, a particle composed of an up, a down, and a strange quark.
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- One of the most promising sandboxes for model builders has been anomalies in B physics, interactions involving B mesons, which are particles composed of a bottom quark or antiquark plus another type of quark. Results from LHCb at CERN, Belle in Japan, and Babar in the US, point to potential problems with the standard model predictions for some rare B meson decays.
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- Alone, each notable B physics result is only a few-sigma discrepancy. But taken together, the aggregate of the results is a 5- to 7-sigma deviation from the standard model estimates.
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- If the anomalies are a hint of something real, the simplest explanation is a new particle called the Z′, a partner to the Z boson that differs only slightly in its interactions with other particles. This hypothetical particle would form a bridge between leptons (electrons, muons, and taus) and quarks.
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- Many theorists attempt to link anomalies together in models. For example, a new anomaly from KOTO, an experiment at JPARC in Japan, measuring the lifetime of neutral kaons, has piqued theorists proposed a light, Higgs-like particle, or scalar boson, that would interact with muons and would explain both the KOTO anomaly and the muon anomaly.
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- While theorists like finding one explanation for multiple anomalies, it’s often difficult to match all the data. Attempts to find a combined explanation for both the B physics and muon anomalies have mostly fallen flat.
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- The best models, according to theorists, are those that fit the data naturally, without too much finagling. Neutrinos have been the focus of several recent anomalies, such as unexpected oscillations in the flavors of neutrinos observed by MiniBooNE at Fermilab in 2018
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- To explain neutrino anomalies, the most straightforward thing to do is to introduce one new neutrino. The trouble is that this addition, called a “sterile neutrino“, is a possible dark matter candidate, which means it must agree with cosmological data. Constraints like this can require highly tailored solutions from theorists.
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- Since the Higgs was discovered in 2012, anomalies have gained currency as some of the only breadcrumbs available for particle physicists to follow on the path toward new physics.
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- Coming up with models that explain minute discrepancies in the data may lack some of the grandeur of trying to solve quantum gravity. But for the time being, anomalies offer fresh challenges to the standard model. So particle physicists will continue hunting down blips in the data and proposing new models to explain the discrepancies, even though the future is uncertain.
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- September 26, 2020 2843
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