So, string theory supports a landscape of possible universes. Physicists have studied that the density of the dark energy offers a stable negative value. This is quite different from our universe that gives a stable positive value.

If we are talking about the string theory, we cannot separate it from the prominent string theorist, Cumrun Vafa of Harvard University and collaborators. From their paper, they created a simple formula to dictate the kinds of universes. There are allowed universes to exist and others are forbidden. But now, Vafa and his team were thinking that in the string landscape, other universes do not exist. If what they thought was correct, it means that the string theory must be wrong.

So, at the same time in Okinawa, Japan, Vafa started his presentation about the conjecture at the Strings 2018 Conference. At this event, many physicists in the worldwide streamed his presentation. No one could stop the debate. Some said it was wrong, others agreed, the remaining physicists in the middle. Of course, they were in a big confusion, but huge excitement. Vafa explained that if this conjecture was right, there will be a lot of tremendous implications for the cosmology.

Wrase, another string theorist has already written two papers. One of his writing is quite possible to become the refinement of the conjecture, but after seeing the Conference, he recalled thinking. It was in June 25, Vafa, Hirosi Ooguri, Georges Obeid, and Lev Spodyneiko posed the conjectured formula. After that, Vafa, Prateek Agrawal, Obeid, and Paul Steinhardt released the second paper. They argued that as the universe expands, the density of energy in the vacuum of empty space can decrease faster than the certain rate. Here, everything appears to be true in all simple string theory-based models of universes. On the other hands, what they have explained may violate two popular beliefs about the actual universe: This is quite impossible to accept the universe’s present day-expansion and the leading model of the explosive birth.

## The Big Question of Dark Energy

Based on the telescope observations since 1998, there was a big indication that the cosmos is expanding faster all the time. It means that the vacuum of empty space must be infused with gravitationally dark energy. Additionally, the amount of dark energy infused in the empty space always constant over time. But this new conjecture also explains that the vacuum energy of the universe must be decreasing.

Vafa and his team argued that universes with stable, constant, positive amounts of vacuum energy, called de Sitter Universes, and that is impossible. If Vafa is right, it means the Sitter universes lie not in the landscape, but in the “swampland”. It is like you think you can construct them, but you cannot. This argument if it is true, it means that the density of the dark energy in this universe cannot be constant, but must take a form known as “quintessence” – the energy source that diminish since tens billions of years.

The discovery of quintessence could revolutionize fundamental physics and cosmology. Physicists, if this argument is true, they need to rewrite the history and the future of cosmology. The quintessent universe is not tearing apart in a Big Rip, but gradually decelerates and in most models would stop expanding and contract in a Big Bounce or a Big Crunch.

## The Inflation

Previously, people widely believed the story how about the universe, the Big Bang theory that is also popular as the cosmic inflation. But, now many of them may doubt due to the new swampland conjecture. This theory explains a minuscule, energy-infused speck of space-time rapidly inflated to create the macroscopic universe we inhabit. But, the problem is the “inflation field” of energy. In fact, the inflation field’s energy would probably have needed to diminish too quickly to create a smooth and flat universe.

In the meantime, a physics professor at Stanford University and a leader to construct string-theoretic models of inflation, Eva Silverstein thinks it can be false. She and her husband suggested a set of stringy ingredients may be used to construct de Sitter universes.

A string theorist and cosmologist at New York University, Matthew Kleban, also work on the strong models of inflation. According to him, the new swampland conjecture is something speculative. It is because the new conjecture has yet to be explored. But, he admits that based on the current evidence, this conjecture can be true.

## Mapping The Swampland

The physicists need more time to see whether the de Sitter swampland conjecture and the future experiments have the power to falsify the string theory. The discovery at the beginning of the 2000s that string theory likely has 10500 solutions killed the dream that might uniquely and inevitably predict the properties of one universe. Likely, this theory could support any observations, but it could be difficult to test or to disprove it based on any experiments.

It was in 2005 Vafa and his team started to think about how to pare the possibilities down by mapping out the fundamental features of nature. For instance, the weak gravity conjecture in any logical universe can be the reason for the universe to get tossed from the landscape into the swampland.

The real challenge to deal with is how the string theory predicts the existence of 10 space-time dimensions. At this time, we only have a 4-D universe. The theorists posit that the six additional spatial dimensions must be small.