Einstein's Ring

-  2127  - Einstein's Ring.  How can empty space bend light?   How can time slow down or speed up?  Is it not the same everywhere in the universe?  If general relativity holds true on the scale of entire galaxies then 95% of the universe is made up of invisible substances called dark matter and dark energy.

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----------------------------------  2127  -  Einstein's Ring

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-  Gravity may be the weakest of the fundamental forces in nature, but its weak attractiveness over long distances allowed mass in the early universe to clump together and form galaxies, stars and planets.

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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 space and time.  This idea also warps one's mind and even thinking.  How can empty space bend liight?   How can time slow down or speed up?  Is it not the same everywhere in the universe? 

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-  If general relativity holds true on the scale of entire galaxies then 95% of the universe is made up of invisible substances called dark matter and dark energy.

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-  The first ever test of general relativity was carried out by Arthur Eddington in 1919. As massive objects bend space-time, light rays should be deflected as they pass the object rather than travelling in a straight line. 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 for an entire galaxy.

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-  The galaxy, E325, located some 450 million light years away, is one of the closest examples of a rare cosmic alignment  sitting directly between us and a second, more distant, galaxy. The background galaxy in this case is some 17 billion 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, or bend,  as they travel through the curved space-time around E325, we see images of it that are slightly distorted from what we would otherwise see.  The effect is called gravitational lensing. The amount of deflection of light passing E325 is about 1 / 1200 of a degree.

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-  If the curvature of space-time near the first galaxy is great enough then multiple images of the background galaxy will form on either side of the lens galaxy. After subtracting the light of the galaxy, a blue Einstein ring becomes visible in the centre.

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-  Astronomers can measure the amount of mass in E325 by looking at 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 centre of mass. More mass in the galaxy means a stronger gravitational force and  the stars orbit faster.

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-  To measure their speed the “Doppler effect” measures the stretching or shortening of light wavelengths caused by the star's velocity coming towards or going away from us. We measure this change in frequency in the light coming from stars to estimate their speed. The light from stars moving towards us is slightly shifted to the blue frequency, and stars moving away are shifted to the red frequency. The faster they move, the bigger the shift.

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-  Because E325 is so distant, it’s not possible to measure the Doppler effect for individual stars. Astronomers instead measured the light from all the stars in a patch and estimated the different velocities using statistical methods. These observations are made using the Very Large Telescope in Chile.

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-  Once the velocities of the stars and the radius of the “Einstein ring” are known the amount of spatial curvature is calculated and compared to the total mass of the galaxy. These measurements were made with a mere 0.9% total uncertainty. This is the most precise test of general relativity over length scales larger than our solar system.

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-  According to these calculations 95% of our universe is made up of dark matter and dark energy. 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 exerting an expansive force on the universe, and explains 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 results as well. They typically tweak how gravity works over long distances so that dark energy isn’t needed to explain the accelerating expansion of the universe..

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-  However,  gravity does behave the way general relativity expects on scales of up to 6,000  light years. Not only does this validate Einstein, it also shows that either dark energy and dark matter are real, or,  general relativity needs to be amended only on length scales that are larger than 6,000  lightyears.

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-  In the next decade, two new telescopes will see their first light,  the Euclid satellite and the Large Synoptic Survey Telescope.  They will be able to detect deviations from general relativity on scales more than 1,000 times longer than probed in E325. 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, all of  Einstein's calculations have been proven to be correct..

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-  October 16, 2018.      

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 ---------------------   October 16, 2018           -------------------------

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