# Why was the flat, no Lambda Universe previously favoured?

In this paper they state that a flat universe with no dark energy (##\Lambda = 0##) was favoured before the measurement results -- as they convey in the paper -- implies that the universe can not be flat and have a zero cosmological constant simultaneously.

What reasons were there, before this measurement, to favour a flat universe with zero ##\Lambda##?

## Answers and Replies

PeterDonis
Mentor
2020 Award
In this paper they state that a flat universe with no dark energy (Λ=0\Lambda = 0) was favoured before the measurement results -- as they convey in the paper -- implies that the universe can not be flat and have a zero cosmological constant simultaneously.
Which specific statement in the paper are you referring to? All I see is a statement in the abstract that the ##\Lambda = 0## flat model is the "simplest inflationary universe model".

Chalnoth
In this paper they state that a flat universe with no dark energy (##\Lambda = 0##) was favoured before the measurement results -- as they convey in the paper -- implies that the universe can not be flat and have a zero cosmological constant simultaneously.

What reasons were there, before this measurement, to favour a flat universe with zero ##\Lambda##?
I don't think such a universe was ever favored by the data. But before the CMB measurements in the mid 90's, our estimates of the total matter density had such huge error bars that it was entirely possible for the total matter density to be high enough to make a flat universe with no cosmological constant. Even back then, the estimates of matter density seemed to be on the low side, but the uncertainty was pretty high.

The supernova estimates in the late 90's, especially combined with the later the WMAP CMB observations, demonstrated beyond all reasonable doubt that our universe was nearly spatially-flat, and the matter density was far too low compared to the expansion rate to make it spatially flat.

The only reasons to believe that the universe was spatially-flat with no cosmological constant before these observations were purely theoretical: many believed that the value of the cosmological constant, approximately $10^{-122}$, was way, way too small to be reasonable (if this value were higher by more than a couple orders of magnitude, no structure would have formed). So people just assumed there must be some symmetry that sets the cosmological constant to zero.

As for spatial flatness, that prediction comes from cosmic inflation: it's difficult to generate a universe with cosmic inflation that explains the horizon problem without also not producing an exponentially-flat universe.

can I ask for comments on Professor Wiltshire's alternative cosmology? He seems to argue that time slows down in structure regions, and passes faster in voids, which temporal inhomogeneity coupled with spatial inhomogeneity could account for observations

Chalnoth