Can String Theory really fail to contain a de Sitter vacua? I was reading a post earlier from Peter Woit's Not Even Wrong blog and came across the following reference to the paper "What if string theory has no de Sitter vacua?" by Ulf H. Danielsson, Thomas Van Riet. The preprint is on the Arxiv here - it isn't clear if it has been published in a refereed journal.

From this analysis we conclude that string theory has not made much
  progress on the problem of the cosmological constant during the last
  15 years. There is a general agreement that the presence of dark
  energy should be an important clue to new physics. So far, string
  theory has not been up to the challenge. Or to be more precise, string
  theorists have not been up to the challenge.
From what we have seen so far, we believe that the most sensible
  attitude is to accept there are no dS vacua at all because string
  theory conspires against dS vacua.
The suggestion here is basically that effective field theories on a
  deSitter background are in the Swampland, so can’t be derived from
  string theory. Since we seem to live in a deSitter space, the obvious
  conclusion to draw from this is that string theory is falsified: it
  can’t be the fundamental theory we are looking for. The authors
  discuss various unconvincing ways to try and avoid this conclusion.

Now I'm well aware that Woit really, really doesn't seem to like String Theory. That being said, the books/papers/videos I've come across over basically my entire life seem to showcase the radical potential for the theory to help us understand the most foundational aspects of our world (replicate standard model, combine quantum mechanics and general relatively, etc.), so this would be pretty shocking to me.
Can anyone explain what might be going on here? Any input is appreciated...
 A: I haven't read the paper by Danielsson and Van Riet, but they seem to be in good company here.
Recently, four prominent string theorists wrote a paper that suggested (even metastable) De Sitter space might actually belong to the swampland (be impossible to realize in string theory): 
https://arxiv.org/abs/1806.08362.
In this paper they formulated a mathematical conjecture that places bounds on the possible local minima in field space:
\begin{equation}
| \nabla V| \geq c \cdot V,
\end{equation}
with $V$ the scalar potential (the gradient is taken in field space) and $c$ apparently of order one. This would mean that at positive $V$ (positive vacuum energy), the scalar potential is never at a local minimum. 
Although the conjecture has not been rigorously proven yet (the authors do provide much evidence for it), it seems that this paper is big news in the string theory community. 
The authors suggest quintessence models as a way out; these are models in which the cosmological constant is actually not constant, but a dynamical field. This has other consequences, which I am not very familiar with; one of the possibilities might be that fundamental 'constants', like the charge of the electron, might also change in time.
A: From a quick scan through the paper it seems there is a hole in the argument. Supersymmetry makes a positive cosmological constant hard to achieve, but supersymmetry is obviously broken at some energy since the universe we see around us isn't supersymmetric. So the fact that string theory is supersymmetry does not necessarily forbid a de Sitter solution.
The authors address this in the paper in the introduction to section 3 where they say:

When SUSY is broken well below the KK scale, one could justify a lower-dimensional effective field description that is a supergravity theory where the dS vacuum breaks supersymmetry spontaneously. This constrains the effective
  action much stronger compared to models that break SUSY at or above the KK scale. We will not discuss this in any detail further on, so let us mention here that the classical vacua typically break SUSY at the KK scale, whereas the quantum IIB vacua", where SUSY is broken by anti-branes for instance should have SUSY broken below the KK scale.

Phenomelogically we have tended to assume supersymmetry is broken at or around the few TeV scale as this helps explain the low mass of the Higgs boson. Unless I have missed something (which is quite possible since I only skimmed the paper) it appears that the authors don't consider this case.
