Einstein's description of gravity just got much harder to beat

Date:
October 1, 2020
Source:
University of Arizona
Summary:
Astrophysicists put general relativity to a new test with black
hole images.



FULL STORY ========================================================================== Einstein's theory of general relativity -- the idea that gravity is matter warping spacetime -- has withstood over 100 years of scrutiny and testing, including the newest test from the Event Horizon Telescope collaboration, published today in the latest issue of Physical Review Letters.


========================================================================== According to the findings, Einstein's theory just got 500 times harder
to beat.

Despite its successes, Einstein's robust theory remains mathematically irreconcilable with quantum mechanics, the scientific understanding of
the subatomic world. Testing general relativity is important because
the ultimate theory of the universe must encompass both gravity and
quantum mechanics.

"We expect a complete theory of gravity to be different from general relativity, but there are many ways one can modify it. We found that
whatever the correct theory is, it can't be significantly different from general relativity when it comes to black holes. We really squeezed down
the space of possible modifications," said UArizona astrophysics professor Dimitrios Psaltis, who until recently was the project scientist of the
Event Horizon Telescope collaboration. Psaltis is lead author of a new
paper that details the researchers' findings.

"This is a brand-new way to test general relativity using supermassive
black holes," said Keiichi Asada, an EHT science council member and an
expert on radio observations of black holes for Academia Sinica Institute
of Astronomy and Astrophysics.

To perform the test, the team used the first image ever taken of the supermassive black hole at the center of nearby galaxy M87 obtained
with the EHT last year. The first results had shown that the size of
the black-hole shadow was consistent with the size predicted by general relativity.



==========================================================================
"At that time, we were not able to ask the opposite question: How
different can a gravity theory be from general relativity and still be consistent with the shadow size?" said UArizona Steward Theory Fellow
Pierre Christian. "We wondered if there was anything we could do with
these observations in order to cull some of the alternatives." The team
did a very broad analysis of many modifications to the theory of general relativity to identify the unique characteristic of a theory of gravity
that determines the size of a black hole shadow.

"In this way, we can now pinpoint whether some alternative to general relativity is in agreement with the Event Horizon Telescope observations, without worrying about any other details," said Lia Medeiros, a
postdoctoral fellow at the Institute for Advanced Study who has been part
of the EHT collaboration since her time as a UArizona graduate student.

The team focused on the range of alternatives that had passed all the
previous tests in the solar system.

"Using the gauge we developed, we showed that the measured size of the
black hole shadow in M87 tightens the wiggle room for modifications
to Einstein's theory of general relativity by almost a factor of
500, compared to previous tests in the solar system," said UArizona astrophysics professor Feryal O"zel, a senior member of the EHT
collaboration. "Many ways to modify general relativity fail at this
new and tighter black hole shadow test." "Black hole images provide a completely new angle for testing Einstein's theory of general relativity,"
said Michael Kramer, director of the Max Planck Institute for Radio
Astronomy and EHT collaboration member.



========================================================================== "Together with gravitational wave observations, this marks the beginning
of a new era in black hole astrophysics," Psaltis said.

Testing the theory of gravity is an ongoing quest: Are the general
relativity predictions for various astrophysical objects good enough
for astrophysicists to not worry about any potential differences or modifications to general relativity? "We always say general relativity
passed all tests with flying colors -- if I had a dime for every time
I heard that," O"zel said. "But it is true, when you do certain tests,
you don't see that the results deviate from what general relativity
predicts. What we're saying is that while all of that is correct, for the
first time we have a different gauge by which we can do a test that's 500
times better, and that gauge is the shadow size of a black hole." Next,
the EHT team expects higher fidelity images that will be captured by the expanded array of telescopes, which includes the Greenland Telescope,
the 12- meter Telescope on Kitt Peak near Tucson, and the Northern
Extended Millimeter Array Observatory in France.

"When we obtain an image of the black hole at the center of our own
galaxy, then we can constrain deviations from general relativity even
further," O"zel said.

Will Einstein still be right, then?

========================================================================== Story Source: Materials provided by University_of_Arizona. Original
written by Mikayla Mace.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Dimitrios Psaltis, Lia Medeiros, Pierre Christian, Feryal O"zel,
Kazunori
Akiyama, Antxon Alberdi, Walter Alef, Keiichi Asada, Rebecca
Azulay, David Ball, Mislav Baloković, John Barrett, Dan
Bintley, Lindy Blackburn, Wilfred Boland, Geoffrey C. Bower,
Michael Bremer, Christiaan D. Brinkerink, Roger Brissenden, Silke
Britzen, Dominique Broguiere, Thomas Bronzwaer, Do-Young Byun,
John E. Carlstrom, Andrew Chael, Chi- kwan Chan, Shami Chatterjee,
Koushik Chatterjee, Ming-Tang Chen, Yongjun Chen, Ilje Cho, John
E. Conway, James M. Cordes, Geoffrey B. Crew, Yuzhu Cui, Jordy
Davelaar, Mariafelicia De Laurentis, Roger Deane, Jessica Dempsey,
Gregory Desvignes, Jason Dexter, Ralph P. Eatough, Heino Falcke,
Vincent L. Fish, Ed Fomalont, Raquel Fraga-Encinas, Per Friberg,
Christian M. Fromm, Charles F. Gammie, Roberto Garci'a, Olivier
Gentaz, Ciriaco Goddi, Jose' L. Go'mez, Minfeng Gu, Mark Gurwell,
Kazuhiro Hada, Ronald Hesper, Luis C. Ho, Paul Ho, Mareki Honma,
Chih-Wei L. Huang, Lei Huang, David H. Hughes, Makoto Inoue,
Sara Issaoun, David J. James, Buell T. Jannuzi, Michael Janssen,
Wu Jiang, Alejandra Jimenez-Rosales, Michael D. Johnson, Svetlana
Jorstad, Taehyun Jung, Mansour Karami, Ramesh Karuppusamy, Tomohisa
Kawashima, Garrett K. Keating, Mark Kettenis, Jae- Young Kim,
Junhan Kim, Jongsoo Kim, Motoki Kino, Jun Yi Koay, Patrick M.

Koch, Shoko Koyama, Michael Kramer, Carsten Kramer, Thomas
P. Krichbaum, Cheng-Yu Kuo, Tod R. Lauer, Sang-Sung Lee, Yan-Rong
Li, Zhiyuan Li, Michael Lindqvist, Rocco Lico, Jun Liu, Kuo
Liu, Elisabetta Liuzzo, Wen- Ping Lo, Andrei P. Lobanov, Colin
Lonsdale, Ru-Sen Lu, Jirong Mao, Sera Markoff, Daniel P. Marrone,
Alan P. Marscher, Iva'n Marti'-Vidal, Satoki Matsushita, Yosuke
Mizuno, Izumi Mizuno, James M. Moran, Kotaro Moriyama, Monika
Moscibrodzka, Cornelia Mu"ller, Gibwa Musoke, Alejandro Mus Meji'as,
Hiroshi Nagai, Neil M. Nagar, Ramesh Narayan, Gopal Narayanan,
Iniyan Natarajan, Roberto Neri, Aristeidis Noutsos, Hiroki Okino,
He'ctor Olivares, Tomoaki Oyama, Daniel C. M. Palumbo, Jongho
Park, Nimesh Patel, Ue-Li Pen, Vincent Pie'tu, Richard Plambeck,
Aleksandar PopStefanija, Ben Prather, Jorge A. Preciado-Lo'pez,
Venkatessh Ramakrishnan, Ramprasad Rao, Mark G. Rawlings, Alexander
W. Raymond, Bart Ripperda, Freek Roelofs, Alan Rogers, Eduardo
Ros, Mel Rose, Arash Roshanineshat, Helge Rottmann, Alan L. Roy,
Chet Ruszczyk, Benjamin R.

Ryan, Kazi L. J. Rygl, Salvador Sa'nchez, David Sa'nchez-
Arguelles, Mahito Sasada, Tuomas Savolainen, F. Peter Schloerb,
Karl- Friedrich Schuster, Lijing Shao, Zhiqiang Shen,
Des Small, Bong Won Sohn, Jason SooHoo, Fumie Tazaki, Remo
P. J. Tilanus, Michael Titus, Pablo Torne, Tyler Trent,
Efthalia Traianou, Sascha Trippe, Ilse van Bemmel, Huib Jan van
Langevelde, Daniel R. van Rossum, Jan Wagner, John Wardle, Derek
Ward-Thompson, Jonathan Weintroub, Norbert Wex, Robert Wharton,
Maciek Wielgus, George N. Wong, Qingwen Wu, Doosoo Yoon, Andre'
Young, Ken Young, Ziri Younsi, Feng Yuan, Ye-Fei Yuan, Shan-Shan
Zhao.

Gravitational Test beyond the First Post-Newtonian Order with the
Shadow of the M87 Black Hole. Physical Review Letters, 2020; 125
(14) DOI: 10.1103/PhysRevLett.125.141104 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201001113616.htm

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