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Physics, Astronomy & Cosmology / Re: Could the universe be smaller than the visible universe?
« on: 03/12/2024 01:06:13 »Quote from: Paul Cotter
Einstein's equations gave two solutionsA brief history of modern cosmology: Around 1915, Einstein produced a field equation describing the cosmos. The calculation generated an (unknown) constant of integration (as integration always does).
- Einstein's original paper omitted mention of this constant. It was mentioned in a later version.
- As usual in mathematics, you try to find some known condition that allows you to identify the value of this integration constant
- Einstein followed the cosmological assumption of his day that the universe was eternal and unchanging, and this "known condition" allowed him to define a value for this constant.
- Around 1925, Belgian priest Georges Lemaitre (perhaps with a Bible-inspired cosmology) saw in these equations the possibility of what we now call a "Big Bang" creation of the universe. Same equation, different range of values for the cosmological constant, and different initial conditions.
- In 1929, astronomer Edwin Hubble saw that the universe was expanding. This confirmed Lemaitre's suggestion. At this point, the cosmological constant was assumed to be zero. Same equation, different value for the cosmological constant.
- In the 1970s, Vera Rubin saw that galaxies rotated in a strange way, and this led to the idea of Dark Matter. Same equation, same value for the cosmological constant, but introducing the concept that the mass of the universe is a combination of "normal" matter, and the dominant "Dark" matter.
- In the 1990s, the accelerating expansion of the universe was discovered, attributed to Dark Energy. Same equation, different value for the cosmological constant.
Einstein's field equation has proved remarkably resilient for over a century.
- New theoretical, computational and experimental techniques for investigating it have repeatedly confirmed it to an incredible accuracy
- With applications including black holes, GPS, gravitational waves, gravitational lensing and frame dragging
- Discoveries in this time have changed the interpretation of various parameters in the equation, but the equation itself has stood the test of time.
- Some very recent satellite measurements of the expansion of the universe have called into question just how constant this "cosmological constant" truly is. We will need more data to confirm if it has changed, and if so, in what manner
- An engineering "law": "All constants are variable" (under different conditions)
https://en.wikipedia.org/wiki/Einstein_field_equations#The_cosmological_constant
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