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Gravity has often been portrayed as an invisible hand pulling an apple down. To astronomers it’s much more; it’s the invisible architect who guides the universe to its massive structure.
Scientists have been observing galaxies that move much faster than expected for decades.
Some cosmologists, such as Patricio Gallardo of the University of Pennsylvania, have started asking the hard questions. Are Newton and Einstein’s laws of gravitation really universal, or could the universe be subject to more complicated rules?
Gallardo, his team and the Atacama Cosmology telescope (ACT) were able to do this.
They used ACT technology, along with Penn researcher Mark Devlin, and an international group, to measure the gravity effects on galaxy clusters that were hundreds of millions light years apart. This is the largest gravity test ever undertaken: it stretches the laws of physics over vast expanses in space.
Their work shows that gravity drops with distance as Newton’s equations predict and Einstein theory.
The model also shows that the gravity acts as predicted on large scales. This also eliminates rival theories like Modified Newtonian Dynamic (MOND), that attempted to explain the motions observed of galaxies by proposing a different law of gravity.
Gallardo said, It is amazing that Einstein’s general theory of relativity has been able to incorporate the Newtonian law of inverse squares.
Newton was inspired by apples and the orbits of planets within our Solar System when he formulated his inverse-square principle (that the gravity diminishes as the distance squared).
As Gallardo points today, the same principle has been tested at scales Newton would have never imagined: galaxy clusters thousands of millions of miles apart.
The motion of more than 200 million galaxies cannot be explained solely by the kabab. Newtonian logic would suggest that stars further from the galaxy center take more time to orbit. The distribution of visible material, however, shows that the galactic edge spins much faster than what we’d expect from the galaxies visible in it. In clusters of galaxies there is a similar anomaly, in which entire galaxies fly through space at a speed that exceeds their size.
Gallardo explains, This is the main puzzle.
“Either the gravity acts differently at very large scales or there is additional matter in the universe that we can’t directly see.”
Scientists used the Atacama Cosmology Telescope to study the gravitational force by studying the cosmic microwave back-ground, a barely visible remnant of the Big Bang, just 380,000 after the universe was created.
The motion of the galaxy clusters is visible in tiny distortions as this light from ancient times passes. The scientists then measured the distortions in hundreds of thousands clusters to find out how much gravity there is for the biggest objects of the universe.
Gravity’s pulling power would be less pronounced with distance if alternative theories such as MOND were true. The results were almost identical to Newton and Einstein.
The result is that gravity alone cannot account for the high velocity of galaxies. This missing mass is made up of dark matter, an unseen particle which holds everything in place.
What is dark matter and why does it have superhero-like qualities when the world works together to resolve this one of cosmology’s biggest conundrums.
As Gallardo puts it, This study confirms dark matter. We still don’t know its composition.
The next generation of observations of the cosmic microwave background will eventually improve gravity tests. The limits of physics will be pushed by further large-scale surveys to improve this test.
Gallardo laughs: Gravity is still a fascinating field in science.
There are many mysteries to solve. It’s an attractive area.
Journal Reference
- P. A.
Gallardo, K. Pardo, O. H.
E. Philcox, N. Battaglia et al. The Kinematic SunyaevZeldovich effect is used to test the Gravitational Force Law at Cosmological Scales. Physical Review Letters.
DOI: 10.1103/rk8v-rcm3
