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-------------------------- 2765 - UNIVERSE - expanding or not?
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- No matter where we look, the same rules apply everywhere in space: countless calculations in physics are based on this basic principle. The physics is the same everywhere if the theories are right.
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- Since the Big Bang, the universe has swollen in size. It was thought that this increase in size was occurring evenly in all directions. Physicists call this "isotropy." Many calculations on the fundamental properties of the universe are based on this assumption.
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- According to new theories some areas in space expand faster than they should, while others expand more slowly than expected.
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- Based on the observation of galaxy clusters. The galaxy clusters emit X-ray radiation that can be collected on Earth. This was done by the satellite-based telescopes Chandra and XMM-Newton. The temperature of the galaxy clusters can be calculated based on certain characteristics of the radiation. Also, their brightness can be measured. The hotter they are, the brighter they glow.
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- In an isotropic universe the further away a celestial object is from us, the faster it moves away from us. From its speed, we can therefore deduce its distance from us, regardless of the direction in which the object lies. That is the theory anyway?
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- This is because the amount of light that reaches the Earth decreases with increasing distance. So, anyone who knows the original luminosity of a celestial body and its distance knows how bright it should shine in the telescope image.
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- It is precisely at this point that scientists have come across discrepancies that are difficult to reconcile with the isotropy hypothesis: that some galaxy clusters are much fainter than expected. Their distance from Earth is probably much greater than calculated from their speed. And for some others, however, the opposite is the case.
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- There are only three possible explanations for this result. Firstly, it is possible that the X-ray radiation, whose intensity we have measured, is attenuated on its way from the galaxy clusters to Earth. This could be due to as yet undiscovered gas or dust clouds inside or outside the Milky Way.
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- In preliminary tests, however, we find this discrepancy between measurement and theory not only in X-rays but also at other wavelengths. It is extremely unlikely that any kind of matter nebula absorbs completely different types of radiation in the same way.
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- A second possibility is that these are groups of neighboring galaxy clusters that move continuously in a certain direction due to some structures in space that generate strong gravitational forces. These would therefore attract the galaxy clusters to themselves and thus change their speed and their derived distance.
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- The third possibility is that the universe is not isotropic after all? What if the galactic distribution is so unevenly distributed that it quickly bulges in some places while it hardly grows at all in other regions? Such an anisotropy could result from the properties of the mysterious "dark energy," which acts as an additional driving force for the expansion of the universe.
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- However, a theory is still missing that would make the behavior of the Dark Energy consistent with the observations.
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- The current study is based on data from more than 800 galaxy clusters, 300 of which were analyzed. The remaining clusters come from previously published studies. The analysis of the X-ray data alone was so demanding that it took several months. The new satellite-based eROSITA X-ray telescope is expected to record several thousand more galaxy clusters in the coming years. At the latest then it will become clear whether the isotropy hypothesis really has to be abandoned.
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- Scientists have known for almost a century that the universe is expanding, meaning the distance between galaxies across the universe is becoming ever more vast every second. But exactly how fast space is stretching, a value known as the “Hubble constant“, has remained stubbornly elusive.
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- Another new measurement for the rate of expansion in the modern universe is suggesting that the space between galaxies is stretching faster than scientists would expect.
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- As more research points to a discrepancy between predictions and observations, scientists are considering whether they may need to come up with a new model for the underlying physics of the universe in order to explain it.
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- The Hubble constant is the cosmological parameter that sets the absolute scale, size and age of the universe; it is one of the most direct ways we have of quantifying how the universe evolves. The jury is still out on whether there is an immediate and compelling reason to believe that there is something fundamentally flawed in our current model of the universe.
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- Their new observations, made using Hubble Telescope, indicate that the expansion rate for the nearby universe is just under 70 kilometers per second per megaparsec (km/sec/Mpc). One parsec is equivalent to 3.26 light-years distance.
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- Kilometers per second per megaparsec can be put in more familiar units. The rate of Universe expansion is 49,300 miles per hour per every million lightyears distance. So a billion light years away the galaxy is receding 1,000 time faster , or, 49,300,000 miles per hour. At the edge of the visible Universe, at 13 billion year distance, it is receding so fast that light will never reach us.
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- The Hubble Telescope measurement is slightly smaller than the value of 74 km/sec/Mpc recently reported using Cepheid variables, which are stars that pulse at regular intervals that correspond to their peak brightness.
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- How many ways have we measured Universe expansion? In 2001 using the Hubble Space Telescope Key Project team measured the value using Cepheid variables as distance markers. Their program concluded that the value of the Hubble constant for our universe was 72 km/sec/Mpc.
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- Other scientists have taken a very different approach, building a model based on the rippling structure of light left over from the big bang, which is called the Cosmic Microwave Background. These measurements allow scientists to predict how the early universe would likely have evolved into the expansion rate astronomers can measure today. Scientists calculated a value of 67.4 km/sec/Mpc, in significant disagreement with the rate of 74.0 km/sec/Mpc measured with other Cepheid stars.
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- Astronomers have looked for anything that might be causing these mismatches. We would like to get the same answers. So, astronomers sought to check their results by establishing a new and entirely independent path to the Hubble Constant using an entirely different kind of star.
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- Certain stars end their lives as a very luminous kind of star called a red giant, a stage of evolution that our own Sun will experience billions of years from now. At a certain point, the star undergoes a catastrophic event called a “helium flash“, in which the temperature rises to about 100 million degrees and the structure of the star is rearranged, which ultimately dramatically decreases its luminosity.
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- Astronomers can measure the apparent brightness of the red giant stars at this stage in different galaxies, and they can use this as a way to tell their distance.
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- The Hubble Constant is calculated by comparing distance values to the apparent recessional velocity of the target galaxies, that is, how fast galaxies seem to be moving away. The team's calculations give a Hubble constant of 69.8 km/sec/Mpc.
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- NASA's upcoming mission, the Wide Field Infrared Survey Telescope (WFIRST), scheduled to launch in the mid-2020s, will enable astronomers to better explore the value of the Hubble constant across cosmic time.
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- WFIRST, with its Hubble-like resolution and 100 times greater view of the sky, will provide a wealth of new Type 1a supernovae, Cepheid variables, and red giant stars to fundamentally improve distance measurements to galaxies near and far.
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- There is another theory that the Earth, solar system, the entire Milky Way and the few thousand galaxies closest to us move in a vast "bubble" that is 250 million light years in diameter, where the average density of matter is half as high as for the rest of the universe.
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- The universe has been expanding since the Big Bang occurred 13.8 billion years ago. This was first proposed by the Belgian physicist Georges Lemaître (1894-1966), and first demonstrated by Edwin Hubble (1889-1953).
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- The American astronomer Hubble discovered in 1929 that every galaxy is pulling away from us, and that the most distant galaxies are moving the most quickly. This suggests that there was a time in the past when all the galaxies were located at the same spot, a time that can only correspond to the Big Bang.
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- This research gave rise to the Hubble-Lemaître law, including the Hubble Constant (H0), which denotes the universe's rate of expansion. The best H0 estimates currently lie around 70 (km/s)/Mpc.
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- The calculation based on the “cosmic microwave background“, which is the microwave radiation that comes at us from everywhere, emitted at the time the universe became cold enough for light to be able to circulate freely, about 370,000 years after the Big Bang.
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- Using the precise data supplied by the Planck space mission, and given the fact that the universe is homogeneous and isotropic, a value of 67.4 is obtained for H0 using Einstein's theory of general relativity to run through the scenario.
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- The second calculation method is based on the supernovae that appear sporadically in distant galaxies. These very bright events provide the observer with highly precise distances, an approach that has made it possible to determine a value for H0 of 74.
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- These two values carried on becoming more precise for many years while remaining different from each other. It didn't take much to spark a scientific controversy and even to arouse the exciting hope that we were perhaps dealing with a 'new physics.
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- Maybe the universe is not as homogeneous , a hypothesis that may seem obvious on relatively modest scales. There is no doubt that matter is distributed differently inside a galaxy than outside one. It is more difficult, however, to imagine fluctuations in the average density of matter calculated on volumes thousands of times larger than a galaxy.
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- If we were in a kind of gigantic 'bubble, where the density of matter was significantly lower than the known density for the entire universe, it would have consequences on the distances of supernovae and, ultimately, on determining H0."
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- All that would be needed would be for this "Hubble bubble" to be large enough to include the galaxy that serves as a reference for measuring distances. By establishing a diameter of 250 million light years for this bubble, the physicist calculated that if the density of matter inside was 50% lower than for the rest of the universe, a new value would be obtained for the Hubble constant, which would then agree with the one obtained using the cosmic microwave background.
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- However the probability that there is such a fluctuation on this scale is one in 20 to one in 5, which means that it is not a theoretician's fantasy.
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- There are a lot of regions like ours in the vast universe. Obviously, we have a lot more to learn. Stay in school.
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- June 23, 2020 2765
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--------------------- Wednesday, June 24, 2020 -------------------------
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