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After the Big Bang, the Universe was full of matter and radiation. It expanded and cooled, and over millions and even billions of years, the overdense regions attracted more and more matter into them, eventually forming stars, galaxies, and clusters of galaxies. A few billion years ago, a new form of energy — dark energy — became important in the Universe, and pushed the distant galaxies and clusters away, causing them to accelerate. One of the greatest puzzles in physics is where this dark energy came from, and why it has the value that it does.

If dark energy were a lot stronger, the Universe would have been driven apart not only before the first stars and galaxies formed, but even before the first stable atoms could form. If it were stronger in the opposite (negative) direction, the Universe would have recollapsed before anything interesting could have formed. The fact that dark energy is as weak as we observe it to be is one of the greatest cosmic coincidences of all, and one that's seemingly necessary for our existence.

In a series of new papers, however, this fine-tuning problem is shown to be less severe than previously thought. It turns out that dark energy might not matter very much at all for allowing life in the Universe.

The dark energy puzzle goes back to 1987, when Nobel Laureate Steven Weinberg wrote a now-famous paper showcasing how small and finely-tuned the cosmological constant would need to be to allow stars, galaxies, and other gravitationally bound states. The argument goes like this:

  1. The value of a cosmological constant in our Universe could, in principle, take on any positive-or-negative value.

  2. If you try and calculate an estimate based on fundamental constants, you get a (mass)4, where the mass, made out of a combination of the constants G, c, and ħ, is ~1019 GeV/c2.

  3. But if the value of dark energy is greater than ±(10-8 GeV/c2)4 or so, you get a Universe that either recollapses (for -) or is driven apart (for +) before any stars or galaxies can form.

  4. Therefore, we must live in a special place, for the Universe to be so finely tuned.

Although this is the commonly-accepted perspective on dark energy for the past 30 years, there are both theoretical and observational reasons to challenge it.

To read more, click here.


Category: Science