Allowed & Forbidden Universes

“On June 25, Timm Wrase awoke in Vienna and groggily scrolled through an online repository of newly posted physics papers. One title startled him into full consciousness.

“The paper, by the prominent string theorist Cumrun Vafa of Harvard and his collaborators, conjectured a simple formula dictating which kinds of universes are allowed to exist and which are forbidden, according to string theory. The leading candidate for a “theory of everything” weaving the force of gravity together with quantum physics, string theory defines all matter and forces as vibrations of tiny strands of energy. The theory permits some 10,500 different solutions: a vast, varied “landscape” of possible universes. String theorists like Wrase and Vafa have strived for years to place our particular universe somewhere in this landscape of possibilities.

a753c7de5e3d0b3c99670530119e8686eee528ea_2200

“But now, Vafa and his colleagues were conjecturing that in the string landscape, universes like ours—or what ours is thought to be like—don’t exist. If the conjecture is correct, Wrase and other string theorists immediately realized, the cosmos must either be profoundly different than previously supposed or string theory must be wrong.

[…]

“The conjectured formula—posed in the June 25 paper by Vafa, Georges Obied, Hirosi Ooguri, and Lev Spodyneiko, and further explored in a second paper released two days later by Vafa, Obied, Prateek Agrawal, and Paul Steinhardt—says, simply, that as the universe expands, the density of energy in the vacuum of empty space must decrease faster than a certain rate. The rule appears to be true in all simple string-theory-based models of universes. But it violates two widespread beliefs about the actual universe: It deems impossible both the accepted picture of the universe’s present-day expansion and the leading model of its explosive birth.

“Since 1998, telescope observations have indicated that the cosmos is expanding ever so slightly faster all the time, implying that the vacuum of empty space must be infused with a dose of gravitationally repulsive “dark energy.”

“In addition, it looks like the amount of dark energy infused in empty space stays constant over time (as best as anyone can tell).

“But the new conjecture asserts that the vacuum energy of the universe must be decreasing…”

Text: The Universe as We Understand It May Be Impossible, The Atlantic.

Pic: Oystein Aspelund, from Dark Brasilia

Advertisements