3806 - DARK ENERGY - new theories are needed?

 

     3806  -   DARK  ENERGY  -  new theories are needed?     In 1916, Einstein finished his Theory of General Relativity, which describes how gravitational forces alter the curvature of spacetime.   This theory predicted that the Universe is expanding, which was confirmed by the observations of Edwin Hubble in 1929.

    

            ---------  3806    -  DARK  ENERGY  -  new theories are needed?

            -   Astronomers have looked farther into space, and hence, back in time, to measure how fast the Universe is expanding.  This is the Hubble Constant of the rate of universe expansion, These measurements have become increasingly accurate thanks to the discovery of the Cosmic Microwave Background (CMB) and observatories like the Hubble Space Telescope.

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            -    Astronomers have traditionally done this in two ways:

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            ------------------  directly measuring it locally (using variable stars

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            -------------------   supernovae) and indirectly based on redshift measurements of the CMB and cosmological models.

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            -    Unfortunately, these two methods have produced different values over the past decade. As a result, astronomers have been looking for a possible solution to this problem, known as the “Hubble Tension.”

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            -  The direct method  of measuring the rate of universe expansion involves using supernovae as “standard candles” (distance markers) to conduct measurements on the local scale.

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            -   The indirect method involves comparing measurements of the CMB with cosmological models, like the Lambda Cold Dark Matter (LCMD) model, which includes the presence of Dark Matter and Dark Energy.

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            -    These two methods produce different results, the former yielding a value of 73 km/s per megaparsec (Mpc) and the latter yielding 67 km/s,

            -      ( Mpc = kilometers per second per 3.26 lightyears distsnce)

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            -  The “Hubble constant” is the present rate at which the Universe expands. The “Hubble tension” is a discrepancy in the value you find for the Hubble constant when you either measure the expansion rate as best you can at present or you predict the value it should have based on the way the Universe looked after the Big Bang coupled with a model of how the Universe should evolve. Its a problem because if these two ways do not agree, it makes us think we are misunderstanding something about the Universe.

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            -    Many candidates have been offered to explain the discrepancy, ranging from:

            -------------------------  The existence of extra radiation, modified General Relativity (GR),

            ------------------------  Modified Newtonian Dynamics (MOND),

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            ------------------------   Primordial magnetic fields,

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            ------------------------  The existence of Dark Matter and Dark Energy during the early Universe that behaved in different ways.

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            -  These can generally be divided into two categories:

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            ----------------------  early-time (shortly after the Big Bang)

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            ----------------------   late-time solutions (more recently in cosmic history).

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            -    “Late-time solutions' postulate that the energy density in the post-recombination Universe,  when the ionized plasma of the early Universe gave rise to neutral atoms (ca. 300 000 years after the Big Bang) is smaller than in the standard LCMB model.

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            -    “Early-time solutions”, meanwhile, postulate that the energy density was somehow increased before recombination occurred so that the “sound horizon” (the comoving distance a sound wave could travel) is decreased.

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            -    After recombination, the energy density would have decayed faster than other forms of radiation, thus leaving the late evolution of the Universe unchanged. It would produce a burst of extra, unexpected expansion in the young Universe that, if we didn’t know about it, would cause the predicted value to underestimate the true value.

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            -    Our current measurements on the CMB are not precise and robust enough yet to distinguish EDE models from the standard LCDM model. What is needed, moving forward, are improved local measurements that will help refine the Hubble Constant and remove any systematic errors. Second, more precise measurements of CMB polarization on smaller angular scales are needed to test EDE and other new physics models.

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            -    As we have learned over and over in cosmology, there is no single bullet  robust conclusions are only reached with multiple observational avenues and a tightly knit web of calibrations, cross-calibrations, and consistency checks.

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            January 1, 2022                            3806                                                                                                                               

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            --------------------- ---  Monday, January 2, 2023  ---------------------------

             

             

             

             

                     

             

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