Our cosmic environment is the result of a delicate balance of cosmic forces—gravity and pressure, cooling and heating, expansion and collapse. The final product, when all these pushes and pulls come into balance, is our Milky Way galaxy, where stars form in a rotating disk of gas and a diffuse halo of dark matter.
Even prior to the observational confirmation of dark energy, Steven Weinberg, the Nobel Prize-winning physicist, had wrestled with the theoretical overestimate and suggested a remarkable solution. He noted that the amount of vacuum energy can vary from place to place, depending on how all the different fields in the universe add and subtract. In our region of space, the amount of vacuum energy is minuscule, requiring all these fields to cancel one another out to an incredible degree. That seems highly improbable, but in any large enough system, even the most improbable event will occur somewhere. In the vast majority of cosmic regions, the excess of dark energy throws off the balance of forces that enables galaxies, stars, planets, and people to form. We see an improbably precise cancellation of known and unknown fields because we couldn’t exist otherwise.
We have been testing Weinberg’s idea in computer simulations, and we’re finding that galaxies are more robust than we thought.
We haven’t yet published our findings, but if they hold up, Weinberg’s argument looks shaky. Suppose there are other regions of the universe with more or less dark energy than we have. Essentially, each region picks a number between zero and 10120, on a scale where our own region picked 1. Now imagine taking your survey clipboard around to check on the lifeforms that evolved in those regions; if you meet one, ask how much dark energy they see. Most regions are dead, of course, which explains why there is no one to tell you, “My region picked 1030 or 1090.” But there are plenty of observers who see 10 or 100 or more, in which case we might wonder why we are the outliers who see a value of 1. If the theory says “expect about 100,” but we see 1, we cannot explain our value as a mere selection effect.
It’s early days. We’re still analyzing. Our computer continues whirring. But, on balance, perhaps we should keep looking at other options to explain dark energy.More.
U Sydney cosmologist Luke Barnes is known for thinking that fine-tuning of the universe should be treated as evidence. That, of course, entails recognizing the category “evidence,” currently under siege.
See also: Free live interactive webinar Saturday with fine-tuning astrophysicist Luke Barnes
Anti-dark energy theories are burnt toast?
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