Not much progress has been made in the past ten years with the dark energy concept and as a practical matter the general population could care less. We have made some advances in describing dark matter but don't understand what it is and like dark energy it's not a topic of conversation appropriate in grocery store check out lines but, I find our cute titles and confusion interesting. Explaining rotational anomalies observed in spiral galaxies as being the result of an additional gravitational force we have applied "in the box" thinking.
Mass generates gravity. Additional gravity explains the phenomena we are observing therefore there must be more mass in and around the galaxies than we can see and light must be passing through it or we would see it's shadow. Instead of leaving the question open to other explanations for an observed angular momentum disparity between the mass of all the visible matter in distant galaxies we immediately assumed a great deal of hidden matter, (which makes our equations work), and call it dark. In fact it isn't dark, it is invisible, (not responsive to light). What we are really looking for is an invisible force causing galaxies to rotate, and clump, in ways we didn't expect and can't explain. It isn't dark and it may not be matter.
Not long after our discovered assumption that a mysterious form of matter dominated the gravitational scene we got a look at light coming in from distant galaxies much further out than we had ever seen before, thanks to the Hubble telescope, and discovered that early galaxies were not separating from each other as fast as nearby galaxies. Ironically Hubble, the astronomer, was the first to use the red shift of light to reach the conclusion that the Universe was expanding and now a space telescope named after him presents us with evidence that it is not only expanding, it is accelerating and suddenly we are faced with another gravitational challenge.
For a long time we thought that the observed expansion, widening separation of galaxies, was the result of an initial push, the big bang, (another cute but misleading title), and that the total mass of all matter in the universe, (I sometimes wonder if the guy who calculated this was related to the guy who added all the begats in the old testament to put creation at about six thousand BC), to come to the conclusion that the initial push, (big bang energy was nearly balanced by gravity's tendency to slow things down and the Universe might slow to a stop and then start to collapse. All of this scientific speculation was based on observations but also on a lot of assumptions that have used the expansion rate, amount of assumed gravity and some heavy mathematics to formulate a history for our Universe leading back 13.8 billion years to a singularity, but now we seem to have found a major imbalance and have explained it with a new cute title, "Dark Energy"
Dark energy, like dark matter, is evidenced only in its effect and has no visible attributes and is the result of us being able to discern faint light never seen before that was emitted by stars 10 or 12 billion years ago, {talk about snail mail). So now we are comparing light that was emitted a few thousand years ago to light emitted billions of years earlier and concluding that the big bang was really just a mini fart compared to whatever is really blowing up the balloon. Hidden in all the formulae that leads us to these almost preposterous conclusions is our misguided use of time as a term with significance beyond comparative movements or change. Relativity is everywhere and everywhen. Time is not a thing.
Our standard for comparing rates and distances is the speed of light we assume it travels at a constant speed through space, (even though we know it slows down traveling through transparent materials and in intense gravity fields). The speed of light is our yardstick for measuring time and distances but what if our yardstick is elastic and stretches as the universe expands. We assume and have calculated a mass total for all matter in the Universe and assume and have evidence that the universe is expanding. Gravity also appears to be a constant directly associated with proximity and mass and the mass of all matter in the Universe appears to be being disbursed, spreading apart, creating a declining proximity. If the early Universe, the Universe sending us photon messages packaged billions of years ago, was less expanded, gravity would be more concentrated, matter in a much close proximity and light, affected by gravity, would have started it's journey at a slower pace and accelerated as the universal gravity sum weakened.
Our elastic yard stick would then have a compressed end in the early universe and a stretched end as it gets closer to our telescopes Remember what spit out the light long ago is long gone We aren't seeing something that exists anymore, only a faint flicker of photons that have been traveling across space for a very long time and if they have not always been traveling at the speed we now observe them to be traveling when they reach us, they could easily lead us a false conclusions; like the universe is filled with dark energy and expanding at an accelerating rate.
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