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---------------------- 2284 - Einstein‘s Theory of gravity?.
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- Our working theory for gravity comes from Albert Einstein’s general theory of relativity, which states that gravity is a consequence of massive objects warping the very fabric of spacetime.
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- Scientists have validated the theory with great precision inside our solar system, but we haven’t been able to do the same on larger distances, like between galaxies. This new study shows that general relativity holds true on the scale of entire galaxies.
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- The findings strengthen the popular view in cosmology that 95% of the universe is made up of invisible substances dubbed dark matter and dark energy , ruling out several other competing theories.
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- The first ever test of general relativity was carried out by Arthur Eddington in 1919. As massive objects bend spacetime, light rays should be deflected as they pass the object rather than traveling in a straight line.
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- Eddington managed to show that this was the case for light bending around the Sun during a solar eclipse. It has taken exactly 99 years for us to do the same test for an entire galaxy far beyond our solar system.
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- The galaxy, E325, located some 450,000,000 light years away, is one of the closest examples of a rare cosmic alignment. It just happens to be sitting directly between us and a second, more distant, galaxy. The background galaxy in this case is some 17,000,000,000 light years further behind. The centers of these two galaxies are aligned to better than one ten-thousandth of a degree.
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- Because light rays from the distant galaxy are deflected as they travel through the curved spacetime around E325, we see images of it that are slightly distorted from what we would otherwise see . This effect is called gravitational lensing. It is like looking at an object through the stem of a wine glass.
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- The deflection of light passing E325 is about 1/1200 of a degree. If the curvature of spacetime near the first galaxy is great enough then multiple images of the background galaxy will form on either side of the lens galaxy when looking at it with a telescope.
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- The distorted image is called an “Einstein ring” and it can tell astronomers how much spatial curvature there is around E325. Astronomers also measured the amount of mass in E325 by measuring how fast the stars are moving in the galaxy. Similar to the Earth orbiting the Sun, the stars in E325 orbit around the galaxy’s center of mass, with gravity holding them in their trajectories. More mass in the galaxy means a stronger gravitational force and so the stars orbit faster.
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- The “Doppler effect” is used to measure their speed towards us or away from us.. The Doppler Effect is created as wavelengths get stretched to wider bandwidths as they travel through expanding space. The amount of stretching of waves is directly due to this motion. If the bandwidth or frequency of the wave is moving towards us it is red shifted. If it is moving away from us it is blue shifted.
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- A similar effect can be experienced with a radar speed camera makes use of the Doppler Effect by detecting the change in the radio frequency from signals bounced off cars to measure their speed. That radar gun behind the bushes can result in your getting a speeding ticket.
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- In a similar way, we measure the change in frequency in the light from stars to estimate their speed. The light from stars moving towards us is slightly shifted to the blue end of the frequency spectrum. The stars moving away are shifted toward the red end of the frequency spectrum.. The faster they move, the bigger the shift.
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- Because E325 is so distant, it is not possible to measure the Doppler effect for individual stars. We instead measured the light from all the stars in a patch of the galaxy and estimate the different velocities using statistical methods. These observations were made using the Very Large Telescope in Chile.
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- The measured velocities of the stars and the radius of the “Einstein ring” are combined to calculate the amount of spatial curvature resulting from the total mass of the galaxy. The calculation was determined to be very accurate and to have a 0.9% total uncertainty.
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- Much of our cosmological understanding comes from interpretation of observations of the universe that depend on general relativity being correct. These calculations tell us that the vast majority of our universe is made up of dark matter and dark energy.
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- Dark matter is needed to explain the observed motions of stars in galaxies. While we can’t see it directly, we can see that it has a gravitational pull on stars. Dark energy is what exerts an expansive force on the universe. This force is needed to explain the fact that the expansion of the universe is speeding up.
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- But, there are alternative theories of gravity that can explain away these mysterious conclusions. They typically change the formulas of how gravity works over long distances so that dark energy isn’t needed to explain the observations.
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- But, our results pose a problem for these alternative theories by showing that gravity does behave the way general relativity expects on scales of up to 6,000 light years. Not only does this result validate Einstein, it also shows that either dark energy and dark matter are real. Or, maybe, general relativity needs to be amended on distance scales that are larger than galaxies. We need more observations to give us more data to analyze and more reviews for me to write.
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- New telescopes are under construction, the Euclid satellite and the Large Synoptic Survey Telescope, that will be able to detect deviations from general relativity on scales more than 1,000 times longer than probed in E325.
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- If general relativity also passes these tests we will know it is the right theory to describe gravity’s effects on the universe as a whole. So far, it is looking good for Einstein.
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- He figured this all out in his head without all this data from astronomical observations. He believed his math. Somehow this math is a language of the Universe. So far, we can trust it until a better theory comes along. Stay tuned. Astronomy is fun stuff.
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- February 25, 2019
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-------------------------- Monday, February 25, 2019 --------------------------
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