On the Origin of Time: The instant Sunday Times bestseller

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According to the Big Bang, the Universe was hotter, denser, more uniform and smaller in the past. It only has the properties we see today because it’s been expanding, cooling, and experiencing the influence of gravitation for so long. Because the wavelength of radiation stretches as the Universe expands, a smaller Universe should have had radiation with shorter wavelengths, meaning it had higher energies and greater temperatures.

The Universe could be expanding today because it was contracting in the past, and will contract again in the future, presenting an oscillating solution.

Sundials and water clocks

Mass and energy are related by the famous equation E = m c 2 {\displaystyle E=mc In a hypertorus model of the Universe, motion in a straight line will return you to your original ... [+] location. If time is like a torus, it may be cyclical in nature, rather than having always existed or coming into existence a finite amount of time ago. We do not, even today, know the origin of time. ESO and deviantART user InTheStarlightGarden

The different ways dark energy could evolve into the future. Remaining constant or increasing in ... [+] strength (into a Big Rip) could potentially rejuvenate the Universe, while reversing sign could lead to a Big Crunch. Under either of those two scenarios, time may be cyclical, while if neither comes true, time could either be finite or infinite in duration to the past. NASA/CXC/M.WeissBut this severely alters our conceptions of how the Universe began. Earlier, I presented you a graph of how the size (or scale) of the Universe evolved with time. The graph displayed the differences between how the Universe would expand if it were dominated by matter (in red), radiation (in blue), or space itself (such as during inflation, in yellow) at early times. However, I wasn't completely honest with you in displaying that graph. James P. Allison, Mahlon DeLong, Michael N. Hall, Robert S. Langer, Richard P. Lifton and Alexander Varshavsky (2014) Hawking then describes Aristotle and Newton's belief in absolute time, i.e. time can be measured accurately regardless of the state of motion of the observer. However, Hawking writes that this commonsense notion does not work at or near the speed of light. He mentions Danish scientist Ole Rømer's discovery that light travels at a very high but finite speed through his observations of Jupiter and one of its moons Io as well as British scientist James Clerk Maxwell's equations on electromagnetism which showed that light travels in waves moving at a fixed speed. Since the notion of absolute rest was abandoned in Newtonian mechanics, Maxwell and many other physicists argued that light must travel through a hypothetical fluid called aether, its speed being relative to that of aether. This was later disproved by the Michelson–Morley experiment, showing that the speed of light always remains constant regardless of the motion of the observer. Einstein and Henri Poincaré later argued that there is no need for aether to explain the motion of light, assuming that there is no absolute time. The special theory of relativity is based on this, arguing that light travels with a finite speed no matter what the speed of the observer is.

Cornelia Bargmann, David Botstein, Lewis C. Cantley, Hans Clevers, Titia de Lange, Napoleone Ferrara, Eric Lander, Charles Sawyers, Robert Weinberg, Shinya Yamanaka and Bert Vogelstein (2013) Special: Stephen Hawking, Peter Jenni, Fabiola Gianotti (ATLAS), Michel Della Negra, Tejinder Virdee, Guido Tonelli, Joseph Incandela (CMS) and Lyn Evans (LHC) (2013) Yifang Wang, Kam-Biu Luk and the Daya Bay team, Atsuto Suzuki and the KamLAND team, Kōichirō Nishikawa and the K2K / T2K team, Arthur B. McDonald and the Sudbury Neutrino Observatory team, Takaaki Kajita and Yōichirō Suzuki and the Super-Kamiokande team (2016) Jeffery W. Kelly, Katalin Karikó, Drew Weissman, Shankar Balasubramanian, David Klenerman and Pascal Mayer (2022)In this chapter, Hawking describes the development of scientific thought regarding the nature of space and time. He first describes the Aristotelian idea that the naturally preferred state of a body is to be at rest, and which can only be moved by force, implying that heavier objects will fall faster. However, Italian scientist Galileo Galilei experimentally proved Aristotle's theory wrong with by observing the motion of objects of different weights and concluding that all objects would fall at the same rate. This eventually led to English scientist Isaac Newton's laws of motion and gravity. However, Newton's laws implied that there is no such thing as absolute state of rest or absolute space as believed by Aristotle: whether an object is 'at rest' or 'in motion' depends on the inertial frame of reference of the observer. Saul Perlmutter and members of the Supernova Cosmology Project; Brian Schmidt, Adam Riess and members of the High-Z Supernova Team (2015) In particular, the patterns and magnitudes of the fluctuations that we've discovered in the modern radiation left over from that early, hot, dense state teach us a number of important properties about our Universe. They teach us how much matter was present in dark matter as well as normal matter: protons, neutrons and electrons. They give us a measurement of the Universe's spatial curvature, as well as the presence of dark energy and the effects of neutrinos. Alim Louis Benabid, Charles David Allis, Victor Ambros, Gary Ruvkun, Jennifer Doudna and Emmanuelle Charpentier (2015) In this chapter, Hawking also covers how the topic of the origin of the Universe and time was studied and debated over the centuries: the perennial existence of the Universe hypothesised by Aristotle and other early philosophers was opposed by St. Augustine and other theologians' belief in its creation at a specific time in the past, where time is a concept that was born with the creation of the Universe. In the modern age, German philosopher Immanuel Kant argued again that time had no beginning. In 1929, American astronomer Edwin Hubble's discovery of the expanding Universe implied that between ten and twenty billion years ago, the entire Universe was contained in one singular extremely dense place. This discovery brought the concept of the beginning of the Universe within the province of science. Currently scientists use Albert Einstein's general theory of relativity and quantum mechanics to partially describe the workings of the Universe, while still looking for a complete Grand Unified Theory that would describe everything in the Universe.