- 3722 - DARK MATTER - versus the expanding universe? Dark Matter and Dark Energy are called “dark” because we do not know what they are. Astronomers have discovered dark matter around galaxies that existed about 12 billion years ago. This is the earliest detection yet of this mysterious substance that dominates the matter in the universe.
-------------- 3722 - DARK MATTER - versus the expanding universe?
- Dark matter in the early universe is less 'clumpy' than predicted by many current cosmological models. This understanding of how galaxies evolve suggest that the fundamental rules governing the universe could have been different when the 13.7 billion-year-old universe was just 1.7 billion years old.
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- Mapping dark matter in the very early universe is done with the “cosmic microwave background” (CMB), a fossil radiation left over from the Big Bang that is distributed throughout the entire universe. It is radiation left over from the Big Bang and has stretched put it wavelength to microwave wavelengths and lower energies.
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- Because light takes a finite time to travel from distant objects to Earth, astronomers see other galaxies as they existed when the observed light left them. The more distant a galaxy, the longer the light has been traveling to us and thus the further back in time we see them, so we see the most distant galaxies as they were billions of years ago, in the infant universe.
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- Dark matter is the mysterious substance that makes up 85% of the total mass of the universe. It doesn't interact with matter and light like the everyday matter that is 15% and made of protons and neutrons that fills stars, planets and us.
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- To 'see' dark matter astronomers must rely on its interaction with gravity. According to Einstein's theory of relativity, objects of tremendous mass cause the curvature of space-time. The larger the cosmic object, the more extreme the warping of space-time it causes.
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- Massive objects like galaxies cause space-time to curve so strongly that light from sources behind a galaxy is curved, just like the path of a marble rolled across the stretched rubber sheet would deviate. This effect shifts the position of the light source in the sky, a phenomenon called “gravitational lensing“.
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- Astronomers can observe how light from a source behind that galaxy is changed as it passes the 'lens galaxy.' The more dark matter a lens galaxy contains the greater the distortion of the light passing it.
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- Because the earliest and most distant galaxies are very faint, as astronomers look deeper into the universe and further back in time, the lensing effect becomes more subtle and more difficult to see. Scientists need both a lot of background sources and a lot of early galaxies to spot lensing by dark matter. This problem has limited the mapping of dark matter distribution to galaxies that are around 8 to 10 billion years old.
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- But the CMB provides a more ancient light source than any galaxy can provide. The CMB is ubiquitous radiation that was created when the universe cooled enough to allow atoms to form, reducing the number of photon-scattering free electrons in a moment cosmologists call 'the last scattering.' The reduction in free electrons allowed photons to travel freely, meaning that the universe suddenly stopped being opaque and became transparent to light.
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- And just like light from other distant sources, the CMB can be distorted by galaxies with dark matter due to gravitational lensing. The combined lensing distortions of a large sample of ancient galaxies with those of the CMB was used to detect dark matter dating back to when the universe was just 1.7 billion years old. And this ancient dark matter paints a very different cosmic picture.
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- For the first time, we were measuring dark matter from almost the earliest moments of the universe. Astronomers can see more galaxies that are in the process of formation than at the present; the first galaxy clusters are starting to form as well. These clusters can be comprised of between 100 and 1,000 galaxies bound to large amounts of dark matter by gravity.
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- The widely accepted current theory is the “Lambda-CDM model” that suggests that tiny fluctuations in the CMB should have resulted in gravity creating densely packed pockets of matter. These fluctuations eventually lead matter to collapse to form galaxies, stars and planets, and should also result in dense pockets of dark matter.
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- The research will continue to collect data to assess whether the Lambda-CDM model conforms to observations of dark matter in the early universe or if the assumptions behind the model need to be revised.
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- The Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) could allow the researchers to look at dark matter even further back in time. Another powerful tool is two decades' worth of observations of supernova explosions. This is a powerful new analysis tool has provided the most accurate accounting of dark energy and dark matter.
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- Dark energy and dark matter are mysterious because despite making up at least 95% of the universe's energy and matter content, they can't be observed directly. The existence of dark energy is inferred from the fact it drives the accelerating expansion of the universe.
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- Dark matter, which does not interact with light and is thus "invisible" is indirectly detected due to its gravitational influence, which literally prevents galaxies from flying apart as they rotate.
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- Astronomy research confirms that the matter-energy content of the universe is made up by around two-thirds dark energy and one-third matter, most of which is in the form of dark matter. They also confirm that the universe has been expanding at an accelerating rate for the last few billion years, and leaves a key disagreement in the rate of this expansion still unresolved.
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- The new study used more than 1,500 supernovas. Astronomers improved their analysis techniques as well as addressing potential sources of error. The technique relies on what astronomers call Type 1a supernovas, which are a type of cosmic explosion that occurs when stellar remnants called white dwarfs accumulate matter from a companion star at a rapid rate, triggering runaway thermonuclear reactions.
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- This variety of cosmic explosions can be so bright that they outshine the light output from every star in their galaxy combined, so astronomers have spotted this type of supernova as much as 10 billion light-years away.
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- Because light takes a finite time to travel to Earth, astronomers are looking back in time as well, in the case of 10 billion light-years, back to when the universe was just one-quarter of its current age. Every Type 1a supernova releases the same amount of light, so astronomers call them "standard candles" and use these events to measure cosmic distances and the expansion rate of the universe.
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- Distance measurements are possible because the brightness of Type 1a supernova light diminishes as it travels. Calculating the expansion of the universe is more complicated; it relies on determining how much the light has been stretched out, or "redshifted," as it travels for billions of years across an expanding universe.
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- The redshift from supernovas at varying distances and thus at different periods in cosmic history reveals how fast the universe was expanding during its different time periods. Two separate teams of scientists in 1998 used observations of distant Type 1a supernovas to calculate that the expansion of the universe was in fact accelerating.
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- Acceleration came as a major shock to physicists, who had assumed that whatever had triggered the initial rapid expansion of the universe, "the Big Bang" , had dissipated and the expansion rate of the universe had slowed.
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- “Dark Energy” became a placeholder name for the cosmic push that is stretching out the very fabric of the universe faster and faster. Dark matter is almost the flip side of this coin, with its gravitational influence helping to hold galaxies together internally as dark energy pushed them apart from each other.
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- The expansion of the universe began speeding up when dark energy began to dominate over the influence of matter and began to drive the universe apart at an ever-increasing rate.
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- The rate of expansion of the universe is called the “Hubble constant“, which is calculated at 45.6 miles per second per megaparsec with only 1.3% uncertainty. This means for every megaparsec, which equals 3.26 million light-years, the analysis estimates that in the nearby universe space itself is expanding at more than 160,000 miles per hour.
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- Another way of measuring the Hubble constant uses the cosmic microwave background (CMB) radiation, a fossil light left over from an event shortly after the Big Bang. However, this approach and the Type 1a supernova approach suggest different values for the Hubble constant.
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- In many ways, this latest “Pantheon+ analysis” is a culmination of more than two decades' worth of diligent efforts by observers and theorists worldwide in deciphering the essence of the expansion of the universe. I hope this review expanded your mind?
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October 29, 2022 DARK MATTER - versus the expanding universe? 3722
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