Sunday, May 28, 2023

4029 - UNIVERSE EXPANDING - how did it start?

 

-    4029  -   UNIVERSE  EXPANDING  - how did it start?   If you could somehow manage to step outside of the universe, what would it look like? Scientists have struggled with this question, taking several different measurements in order to determine the geometry of the unierse and whether or not it will come to an end.


-------------   4029  -  UNIVERSE  EXPANDING  - how did it start?

-    How do they measure the shape of the universe?

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-   According to Einstein's theory of General Relativity, space itself can be curved by mass. As a result, the density of the universe, or,  how much mass it has spread over its volume, determines its shape, as well as its future.

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-    Scientists have calculated the "critical density" of the universe. The critical density is proportional to the square of the Hubble constant, which is used in measuring the expansion rate of the universe. Comparing the critical density to the actual density can help scientists to understand the cosmos.

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-   If the actual density of the universe is less than the critical density, then there is not enough matter to stop the expansion of the universe, and it will expand forever. The resulting shape is curved like the surface of a saddle. This is known as an “open universe”.

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-    The shape of the universe depends on its density. If the density is more than the critical density, the universe is closed and curves like a sphere; if less, it will curve like a saddle. But if the actual density of the universe is equal to the critical density, as scientists think it is, then it will extend forever like a flat piece of paper.

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-    If the actual density of the universe is greater than the critical density, then it contains enough mass to eventually stop its expansion. In this case, the universe is “closed and finite”, though it has no end, and has a “spherical shape”.

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-    Once the universe stops expanding, it will begin to contract. Galaxies will stop receding and start moving closer and closer together. Eventually, the universe will undergo the opposite of the Big Bang, often called the "Big Crunch." This is known as a “closed universe”.

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-   However, if the universe contains exactly enough mass to eventually stop the expansion, the actual density of the universe will equal the “critical density”. The expansion rate will slow down gradually, over an infinite amount of time. In such a case, the universe is considered flat and infinite in size.

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-    Measurements indicate that the universe is “flat”, suggesting that it is also infinite in size. The speed of light limits us to viewing the volume of the universe visible since the Big Bang; because the universe is approximately 13.8 billion years old, scientists can only see 13.8 billion light-years from Earth.

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-    While studying distant galaxies in the early 20th century, astronomer Edwin Hubble realized that they all seemed to be rushing away from the Milky Way. He announced that the universe was expanding in all directions. Since then, astronomers have relied on measurements of supernova and other objects to refine calculations of how quickly the universe is expanding.

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-   Incomprehensible as it sound, inflation poses that the universe initially expanded far faster than the speed of light and grew from a subatomic size to a golf-ball size almost instantaneously.

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-    Other instruments measure the background radiation of the universe in an effort to determine its shape. NASA's Wilkinson Microwave Anisotropy Probe (WMAP) measured background fluctuations in an effort to determine whether the universe is open or closed. In 2013, scientists announced that the universe was known to be flat with only a 0.4 percent margin of error.

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-    There's a hole in the story of how our universe came to be. First, the universe inflated rapidly, like a balloon. Then, everything went boom.  But how those two periods are connected has eluded physicists. Now, a new study suggests a way to link the two epochs.

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-   In the first period,  the universe grew from an almost infinitely small point to nearly an octillion (that's a 1 followed by 27 zeros) times that in size in less than a trillionth of a second. This inflation period was followed by a more gradual, but violent, period of expansion we know as the Big Bang. During the Big Bang, an incredibly hot fireball of fundamental particles, such as protons, neutrons and electrons, expanded and cooled to form the atoms, stars and galaxies we see today.

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-    The Big Bang theory, which describes cosmic inflation, remains the most widely supported explanation of how our universe began, yet scientists are still perplexed by how these wholly different periods of expansion are connected. To solve this cosmic mystery, a team of researchers at MIT and the Netherlands' Leiden University simulated the critical transition between cosmic inflation and the Big Bang, a period they call "reheating."

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-    The post-inflation reheating period sets up the conditions for the Big Bang and, in some sense, puts the 'bang' in the Big Bang.   When the universe expanded in a flash of a second during cosmic inflation, all the existing matter was spread out, leaving the universe a cold and empty place, devoid of the hot soup of particles needed to ignite the Big Bang.

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-    During the reheating period, the energy propelling inflation is believed to decay into particles.  Once those particles are produced, they bounce around and knock into each other, transferring momentum and energy.  And,  that's what thermalizes and reheats the universe to set the initial conditions for the Big Bang.

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-     Scientists think these hypothetical particles, similar in nature to the Higgs boson, created the energy field that drove cosmic inflation. Their model showed that, under the right conditions, the energy of the inflatons could be redistributed efficiently to create the diversity of particles needed to reheat the universe.

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-    The transition from the cold inflationary period to the hot period is one that should hold some key evidence as to what particles really exist at these extremely high energies.

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-    One fundamental question that plagues physicists is how gravity behaves at the extreme energies present during inflation. In Albert Einstein's theory of general relativity, all matter is believed to be affected by gravity in the same way, where the strength of gravity is constant regardless of a particle's energy.

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-     However, because of the strange world of quantum mechanics, scientists think that, at very high energies, matter responds to gravity differently.   Their model tweaked how strongly the particles interacted with gravity. They discovered that the more they increased the strength of gravity, the more efficiently the inflatons transferred energy to produce the zoo of hot matter particles found during the Big Bang.

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-    Now, they need to find evidence to buttress their model somewhere in the universe.  Astronomers earliest glimpse of the universe is a bubble of radiation left over from a few hundred thousand years after the Big Bang, called the cosmic microwave background (CMB).

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-    Yet the CMB only hints at the state of the universe during those first critical seconds of birth. Future observations of gravitational waves will hopefully provide the final clues.

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May 28, 2023     UNIVERSE  EXPANDING  - how did it start?        4029                                                                                                                       

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