The theory of Inflation explains the apparent consistency of the universe by proposing that the early universe grew exponentially for a 1E-36 seconds. Isn't a simpler explanation that the universe is just older and so the homogeneousness comes from a slower more steady growth? Is there any evidence that rules out a slow growing universe and supports Inflation theory?

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    $\begingroup$ The question seems to be more about observation, not theory, but the biggest problems with inflation are theoretical. Paul Steinhardt has a pretty cogent critique. See Paul Steinhardt, "The Inflation Debate," Sci Am, Apr 2011, p. 38, and pirsa.org/11070029 , notes here physicsforums.com/showthread.php?p=3404021#post3404021 . Basically the critique is that inflation rewards procrastinating "rogue" regions that don't stop inflating, and this leads to a lack of predictive power. Arguably inflation requires more fine-tuning than no inflation (i.e., its probability is lower). $\endgroup$
    – user4552
    Aug 11, 2011 at 14:45

5 Answers 5


As pointed out by Weinberg in his book Cosmology (note, this is NOT Gravitation and Cosmology. He also has a book of that name), inflation was proposed to explain 3 problems: 1)Horizon problem 2)Flatness problem 3)Monopole problem

1)Horizon problem: The evolution of the scale factor before and after decoupling is $\sqrt{t}$ and $t^{\frac{2}{3}}$. We compute the linear dimension of the forward and backward lightcones at the time of decoupling in the hot big bang model. The radius of this light cone is the physical size of the region on the last scattering surface from which we receive the CMB. The backward lightcone is $l_{B} \approx 3(t_{dec}^{2}t_{0})^{1/3}$ ($t_{0}$ is present time.). The forward lightcone radius is $l_{F} = 2t_{dec}$. The ratio $R \equiv \frac{l_{B}}{l_{F}} \approx 70$. The physical wavelength associated with cosmological perturbations grows faster than the Hubble radius as we go back in time. If, a causal mechanism is responsible for the inhomogenities, then these scales should be inside Hubble scale in very early universe. This is possible if, the perturbation associated wavelength decreases faster than Hubble radius as we go back in time. So, $-\frac{d}{dt}\left( \frac{\lambda}{d_{H}}\right) <0 $ ($d_{H}$ is the Hubble radius). This leads to $\ddot{a} >0$. In most cases, we model it as a single scalar field which causes this inflation in a de Sitter background ($\Lambda$ dominated universe)

2)Flatness problem: A less convincing argument of inflation. Experimentally, we observe a vanishing spatial curvature parameter $\Omega_{K} = -\frac{K}{a^2 H^2} = -\frac{K}{a^2}$. In solving this problem, we assume that nothing much happens to the cosmic scale factor and expansion rate from the end of inflation to the beginning of the radiation dominated era i.e $a_{Inflation}H_{Inflation} \approx a_{rad. domination}H_{rad. domination}$. The small value of $|K|/\dot{a}^2$ could be explained by taking $K=0$ i.e a spatially flat universe. However, inflation opens up the possibility that the universe is not at all homogenous and isotropic and that its apparent flatness of the cosmic metric is just the result of inflation.

3)Monopole problem: Standard Model predicts that in a hot early universe, a large number of monopoles must be produced by symmetry breaking from some single gauge theory since it is at an energy scale of about $M = 10^{16}$ GeV. Those monopoles should have persisted even to the present days. However, that is not the case.

Amongst all the above problems, the horizon problem is the most serious one. Since, the other two can be explained by other mechanisms. Also, any number of $e$-foldings not only solves the horizon problem but also the flatness problem and the monopole problem.

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    $\begingroup$ "Standard Model predicts that in a hot early universe, a large number of monopoles must be produced by symmetry breaking from some single gauge theory[...]" Huh? The standard model doesn't have monopoles, does it? $\endgroup$
    – user4552
    Jul 26, 2013 at 4:25

I can not go into much detail here but let me say that exponential growth brings many things that we see around us right now: absence of magnetic monopoles, a homogeneous universe in which no section is a "preferred" section i.e. has more matter density, and many more observable quantities.

Actually after a brief search I found a wiki article stating most of the things i said above and much, much more: http://en.wikipedia.org/wiki/Cosmic_inflation#Observational_status

  • $\begingroup$ And it should also be mentioned that while the universe is quite homogenous globally it also contains local inhomogeneities that can't very well be explained otherwise than by an enormous expansion of quantum fluctuations in the early universe. And the observed data actually matches the inflation calculations. $\endgroup$
    – Marek
    Nov 17, 2010 at 23:56

Personally I think the standard arguments are not yet conclusive. The horizon problem, or the homogeneity problem, can be explained by assuming that the initial condition is homogeneous, without assuming that causal contact in the early universe has smoothed out inhomogeneities. You may object that a homogeneous initial condition is "unnatural", but since we know so little about the big bang singularity, there's nothing conclusive that can be said. The monopole problem is only a problem if you think monopoles exist, which has no empirical evidence so far. The curvature problem, again, is a "naturalness" problem, but we lack a precise definition of naturalness given our inability to understand the big bang singularity.


The main (and original) reason for the proposal of inflationary theory was the horizon problem. That is, the fact the the universe is so incredibly homogeneous and isotropic despite the fact that some parts of the universe are apparently too far away to have exchanged energy. Inflation in the early universe is a powerful explanation for this intriguing observation. Also of note is the flatness problem, which inflation also helps address.

There are of course various other theories to explaining this problems, not in any way related to inflation, such as the varying speed of light (VSL) theory. These are however under active research and still not widely accepted.


The evidence is against:
CMB WMAP maps are inconsistent with the standard cosmological model
, from here:

Large-angle anomalies in the CMB (open access) and in arXiv - April 2010 (my bold)

...We discuss these findings in relation to expectation from standard inflationary cosmology ...
The study of alignments in the low ℓ CMB has found a number of peculiarities. We have shown that the alignment of the quadrupole and octopole planes is inconsistent with Gaussian, statistically isotropic skies at least at the 99% confidence level. Further a, number of (possibly related) alignments occur at 95% confidence levels or greater. Putting together these provides a strong indication that the full-sky CMB WMAP maps are inconsistent with the standard cosmological model at the large-angles. Even more peculiar is the alignment of the quadrupole and octopole with solar system features (the ecliptic plane and the dipole).

In this regard, it is worth noting that our record at predicting the gross properties of the universe on large scales from first principles has been rather poor. According to the standard concordance model of cosmology, over 95% of the energy content of the universe is extraordinary—dark matter or dark energy whose existence has been inferred from the failure of the Standard Model of particle physics plus General Relativity to describe the behavior of astrophysical systems larger than a stellar cluster—while the very homogeneity and isotropy (and inhomogeneity) of the universe owe to the influence of an inflaton field whose particle-physics-identity is completely mysterious even after three decades of theorizing.

Conclusions ... The CMB is widely regarded as offering strong substantiating evidence for the concordance model of cosmology. Indeed the agreement between theory and data is remarkable—the patterns in the two-point correlation functions (TT, TE, and EE) of Doppler peaks and troughs are reproduced in detail by fitting with only six (or so) cosmological parameters. This agreement should not be taken lightly; it shows our precise understanding of the causal physics on the last scattering surface. Even so, the cosmological model we arrive at is baroque, requiring the introduction at different scales and epochs of three sources of energy density that are only detected gravitationally—dark matter, dark energy and the inflaton. This alone should encourage us to continuously challenge the model and probe the observations particularly on scales larger than the horizon at the time of last scattering.

At the very least, probes of the large-angle (low-ℓ) properties of the CMB reveal that we do not live in a typical realization of the concordance model of inflationary ΛCDM.

On the theoretical side: from Michael S. Turner (1997)
Ten Things Everyone Should Know About Inflation

Cold dark matter, which is an important means of testing inflation, is a ten-parameter theory, $h,\Omega_Bh^2,S,n,dn/d \ln{k}, T/S, nT, > \Omega_\nu,g_*, \Omega_\Lambda$ . While this is a daunting number of parameters, especially for a cosmological theory, there is good reason to believe that within ten years the data will overdetermine these parameters. Crucial to achieving this goal are the high-precision, high-resolution measurements of CBR anisotropy that will be made over the next decade by... ...ΛCDM is consistent with all the observations discussed here as well as others;...
Inflation Makes Three Robust Predictions
1 Flat universe
2 Nearly scale-invariant spectrum of gaussian density perturbations
3 Nearly scale-invariant spectrum of gravitational waves

The future: (obviously it is my own perspective)
In this novel cosmological model A self-similar model of the Universe unveils the nature of dark energy the Inflation is not needed.
I had a part in the paper, not as an author, and I appreciate any criticism

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    $\begingroup$ Thank you for the references. But, it should be noted that neither of the first two papers criticising inflation was published in a leading journal, so it might not represent scientific consensus. $\endgroup$ Aug 10, 2011 at 15:11
  • $\begingroup$ @mtrencseni the first link is a regular peer-review publisher. Beeing open-access is not the same as low quality.Some papers, even from large affiliated teams, that deal with inconvenient data had been published in obscure locations; I wonder why: I envision similar problems to peer-review of climate research. Dark interests may be hiding behind the scene. The other paper (about a 'scaling universe') is unpublishable because it is against mainstream beliefs (consensus), but it is also mainstream as old as Newton and Coulomb laws are, and the argument is so simple that you can understand it. $\endgroup$ Aug 10, 2011 at 16:25
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    $\begingroup$ I didn't say it's not peer-reviewed, I said it's not a leading journal. $\endgroup$ Aug 10, 2011 at 16:34
  • $\begingroup$ The vixra paper is pure crackpottery. The other two papers do not support your statement that "the evidence is against." $\endgroup$
    – user4552
    Aug 10, 2011 at 17:02
  • $\begingroup$ @Crowell Then in your oppinion what are the possible interpretations of: a strong indication that the full-sky CMB WMAP maps are inconsistent with the standard cosmological model ? $\endgroup$ Aug 10, 2011 at 18:27

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