"Perhaps the past, if looked upon with care and hindsight, may teach us where we possibly took a wrong turn"

What energy is

If you ask what is energy? some people will tell you that energy is an abstract thing. This was how Richard Feynman described it in the Feynman lectures, volume I chapter 4. He used the analogy of children’s blocks. He said these blocks were absolutely indestructible and could not be divided. But then he said there are no blocks. Only then he contradicted himself by saying energy has a number of different forms, such as gravitational energy, kinetic energy, and heat energy: Image from Assignment point…

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The fate of the universe

One of the things cosmologists like to talk about is the fate of the universe. Some say it all depends on the density parameter omega: Ω. This started life as the average matter density of the universe divided by a “critical” matter density for the Friedmann universe: Based on the Friedmann universes public domain image by BenRG, see Wikipedia Commons and Wikipedia Nowadays when we talk about omega we don’t restrict ourselves to matter alone. That’s because energy doesn’t necessarily take the form of matter, and…

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The edge of the universe

When you dig into the history of cosmology, some things catch your eye. Things like the “boundary conditions” in Einstein’s 1917 cosmological considerations in the general theory of relativity. Or something Willem de Sitter said in his 1917 paper On the relativity of inertia. Remarks concerning Einstein's latest hypothesis. He said this: “if the gμν at infinity are zero of a sufficiently high order, then the universe is finite in natural measure”. There’s also something Paul Steinhardt said in his 1982 Natural Inflation paper. He said this:…

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Dark energy

Dark energy is said to be a mystery. Google on dark energy mystery and you can find plenty of material saying as much. Such as dark energy: the biggest mystery in the universe by Richard Panek in the Smithsonian magazine. Or dark energy: mystery of the millennium by Thanu Padmanabhan on the arXiv. Dark energy comes above dark matter in 10 greatest unsolved mysteries in physics on IFL science, and since it’s circa 68% of the mass-energy of the universe as opposed to 27% for dark…

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Inflation

The theory of cosmic or cosmological inflation is related to Big Bang theory. It’s been around now for the thick end of forty years. Or more. By 1980 a lot of cosmologists were happy that the universe was expanding, and that it wasn’t a steady-state universe. A lot of cosmologists also thought of the initial expansion of the universe as something fairly steady. However inflation introduced the idea that the initial expansion was very rapid: Image from Rod Nave’s hyperphysics As for how it all began,…

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The Big Bang

Big Bang cosmology arguably started in 1917. Vesto Slipher had measured 21 galactic redshifts by 1917. That’s when Albert Einstein wrote his cosmological considerations paper and Willem de Sitter came up with the de Sitter universe. The next year in 1918 Erwin Schrödinger came up with his cosmic pressure. In 1922 Alexander Friedmann came up with a non-static universe. In 1924 he came up with negative and positive curvature, and Knut Lundmark came up with an expansion rate within 1% of measurements today. In 1927 Georges…

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A compressed prehistory of dark energy

In the Wikipedia dark energy article you can read how the term "dark energy" was coined by Michael Turner in 1998. This was in a paper co-authored with Dragan Huterer called prospects for probing the dark energy via supernova distance measurements. It referred to the type 1a supernovae observations made by the Supernova Cosmology Project and by the High-Z Supernova Search Team. Their observations indicated that the expansion of the universe is accelerating, which was contrary to expectations and led to Nobel prizes in 2011: Expanding…

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Dark matter

There’s plenty of evidence for dark matter, ranging from velocity dispersion, flat galactic rotation curves, and gravitational lensing. The evidence for dark matter is so good we can even map it out: Image credit NASA, ESA and R. Massey (California Institute of Technology), see spacetelescope.org There’s also plenty of papers and articles about dark matter candidates. Maxim Khlopov refers to WIMPs, axions, neutrinos, mirror-world particles, extra-dimensional particles, and black holes. Andreas Ringwald refers to neutralinos, gravitinos, sterile neutrinos, and axions. Kim Griest refers to MACHOs, molecular…

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Dark matter candidates

Everybody loves a mystery, and one of the best mysteries in physics is dark matter. As to where it starts, check out Brian Koberlain who said its origins can be traced to the 1600s. Or Alexis Bouvard who in 1821 said anomalies in the orbit of Uranus could be caused by dark matter. Or see the Ars Technica history of dark matter article by Stephanie Bucklin. She said in 1884 Lord Kelvin concluded that “many of our supposed thousand million stars, perhaps a great majority of…

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Firewall!

The black hole firewall is a relatively recent idea. On Wikipedia you can read how it “was proposed in 2012 by Ahmed Almheiri, Donald Marolf, Joseph Polchinski, and James Sully as a possible solution to an apparent inconsistency in black hole complementarity”. Their proposal is known as the AMPS firewall, and the title of their paper is black holes: complementarity or firewalls? They cannot all be true They start by saying “we argue that the following three statements cannot all be true: (i) Hawking radiation is…

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