Explaining and Ordering the Heavens is an online exhibition from The Library of Congress, examining evolving views of the universe over 8 centuries.
I’ve reached the cosmology part of my General Relativity (GR) course, and one of the early points that comes up is my traditional rant against confusing three very distinct concepts when thinking about the universe. Roughly stated, these are; What is the shape of the universe? Is the universe finite or infinite? and Will the universe expand forever or recollapse.
When we apply GR to cosmology, we make use of the simplifying assumptions, backed up by observations, that there exists a definition of time such that at a fixed value of time, the universe is spatially homogeneous (looks the same wherever the observer is) and isotropic (looks the same in all directions around a point). We then specialize to the most general metric compatible with these assumptions, and write down the resulting Einstein equations with appropriate sources (regular matter, dark matter, radiation, a cosmological constant, etc.). The solutions to these equations are the famous Friedmann, Robertson-Walker spacetimes, describing the expansion (or contraction) of the universe.
It is important to take a moment to emphasize what we have done here. GR is indeed a beautiful geometric theory describing curved spacetime. But practically, we are solving differential equations, subject to (in this case) the condition that the universe look the way it does today. Differential equations describe the local behavior of a system and so, in GR, they describe the local geometry in the neighborhood of a spacetime point.
Excerpt of an article written by Mark Trodden at DISCOVER. Continue HERE
Multiverse by BellaCielo
You are special. Don’t worry, this is not the start of yet another Joel Osteen sermon. I mean only that your existence, itself a wildly improbable fact, increasingly seems to be the only peg on which cosmologists can hang the existence of our Universe.
Oh, and not just you, by the way. I’m special, too. All of us observers capable of wondering why we are here are special, because we contribute to what is known as the Anthropic Principle. Here’s the nub of it, given by Stephen Hawking and a colleague in 1973: “The answer to the question ‘why is the universe [the way it is]?’ is ‘because we are here.'”
There’s something odd about that. As the cosmologist George F.R. Ellis notes, the Anthropic Principle sends the arrow of causation winging, feathers first, back to the bow. It declares, to paraphrase DesCartes, “I think, therefore the Multiverse.”
Written by Clay Farris Naff at the Huffington Post. Continue HERE
What existed before the big bang? What is the nature of time? Is our universe one of many? On the big questions science cannot (yet?) answer, a new crop of philosophers are trying to provide answers.
Last May, Stephen Hawking gave a talk at Google’s Zeitgeist Conference in which he declared philosophy to be dead. In his book The Grand Design, Hawking went even further. “How can we understand the world in which we find ourselves? How does the universe behave? What is the nature of reality? Where did all this come from? Traditionally these were questions for philosophy, but philosophy is dead,” Hawking wrote. “Philosophy has not kept up with modern developments in science, particularly physics.”
In December, a group of professors from America’s top philosophy departments, including Columbia, Yale, and NYU, set out to establish the philosophy of cosmology as a new field of study within the philosophy of physics. The group aims to bring a philosophical approach to the basic questions at the heart of physics, including those concerning the nature, age and fate of the universe. This past week, a second group of scholars from Oxford and Cambridge announced their intention to launch a similar project in the United Kingdom.
One of the founding members of the American group, Tim Maudlin, was recently hired by New York University, the top ranked philosophy department in the English-speaking world. Maudlin is a philosopher of physics whose interests range from the foundations of physics, to topics more firmly within the domain of philosophy, like metaphysics and logic.
Yesterday I spoke with Maudlin by phone about cosmology, multiple universes, the nature of time, the odds of extraterrestrial life, and why Stephen Hawking is wrong about philosophy.
Written by Ross Andersen. Interview with Tim Maudlin HERE