* Astronomy

In a classical view of the world, space and time are smooth. The minimum scales at which, according to quantum mechanics, the smoothness breaks down the Planck length and time can be derived from other quantities, but they have not been tested experimentally, nor would they be, given their impossibly small size.

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Title: The arrow of time and the Weyl group: all supergravity billiards are integrable
Authors: Pietro Fré, Alexander S. Sorin

In this paper we show that all supergravity billiards corresponding to sigma-models on any U/H non compact-symmetric space and obtained by compactifying supergravity to D=3 are fully integrable. The key point in establishing the integration algorithm is provided by an upper triangular embedding of the solvable Lie algebra associated with U/H into SL(N,R) which always exists. In this context we establish a remarkable relation between the arrow of time and the properties of the Weyl group. The asymptotic states of the developing Universe are in one-to-one correspondence with the elements of the Weyl group which is a property of the Tits Satake universality classes and not of their single representatives. Furthermore the Weyl group admits a natural ordering in terms of L(T), the number of reflections with respect to the simple roots and the direction of time flows is always towards increasing L(T), which plays the unexpected role of an entropy.

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It is the invisible presence that governs your world. Trailing you like an unshakeable shadow, it ticks and tocks incessantly - you can sense it in your heartbeat, in the rising and setting of the sun, and in your daily rush to make meetings, trains and deadlines. It brings order to our lives through the categories of past, present and future.
Time. There is nothing with which we are so familiar, and yet when you try to pin it down you find only a relentless torrent of questions. Why does time appear to flow? What makes it different from space? What exactly is it? It's enough to make your neurons misfire, then sizzle and smoke.
You are not alone. Physicists have long struggled to understand what time really is. In fact, they are not even sure it exists at all. In their quest for deeper theories of the universe, some researchers increasingly suspect that time is not a fundamental feature of nature, but rather an artefact of our perception. One group has recently found a way to do quantum physics without invoking time, which could help pave a path to a time-free "theory of everything". If correct, the approach suggests that time really is an illusion, and that we may need to rethink how the universe at large works.

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It can drag or it can race, but what if time stopped altogether? It now seems that time could disappear from our universe - and we may already have found evidence of its forthcoming demise.
When astronomers observed a decade ago that supernovae are apparently spreading apart faster as the universe ages, they assumed that something must be causing the expansion of the universe to speed up. But so far, nobody has been able to explain where the "dark energy" causing this acceleration comes from.
Now José Senovilla at the University of the Basque Country in Bilbao, Spain, and his colleagues have a radical answer - we are fooled into thinking that the expansion of the universe is accelerating, because time itself is slowing down.

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