The Cosmological Evidence – 25 Years Ago

Today Facebook reminded me that the picture below is now 25 years old. I have posted it before and it has done the rounds at a number of cosmology conferences (usually to the accompaniment of lots of laughter), but I thought I’d circulate again as a bit of nostalgia and also to embarrass all concerned with this image. The picture was taken at a graduate school in cosmology in Leiden (in The Netherlands) in July 1995. In my memory that was a sweltering hot summer, which is my excuse for the informality of my attire.

Anyway, various shady characters masquerading as “experts” were asked by the audience of graduate students at a summer school to give their favoured values for the cosmological parameters. from from top to bottom these are:

  • the Hubble constant H0;
  • density parameter Ω0 (not split into dark matter and `ordinary’  matter as is now customary);
  • cosmological constant Λ0,
  • curvature parameter k
  • and age of the Universe t0.


From left to right we have Alain Blanchard (AB), Bernard Jones (BJ, standing), John Peacock (JP), me (yes, with a beard and a pony tail – the shame of it), Vincent Icke (VI), Rien van de Weygaert (RW) and Peter Katgert (PK, standing). You can see on the hi-tech digital display screen blackboard that the only one to get anywhere close to correctly predicting the parameters of what would become the standard cosmological model was, in fact, Rien van de Weygaert. Actually he was the only one of us to include a non-zero cosmological constant. My own favourite model at the time was a low-density model with negative spatial curvature.

Nobody is suggesting that panel discussions are the right way to settle scientific questions, of course, but it is interesting to see the diversity of opinions that were around in 1995.

P.S. Note that not all the combinations of parameters presented there are consistent with a Friedman model, but nobody said they had to be!


21 Responses to “The Cosmological Evidence – 25 Years Ago”

  1. V interesting snapshot of history. I think people like Jim Peebles, Sean Carroll , Michael Turner and Lawrence Krauss had been arguing for a non-zero cc throughout the 1990s, isn’t that right?

    • telescoper Says:

      It was clear from about 1990 (I think) that Omega was in the range 0.2 to 0.4. Those who really liked models with flat spatial sections then favoured Lambda. The direct evidence for k=0 (CMB) and Lambda (SN1a) did not really come until 2000ish.

      • Phillip Helbig Says:

        It was that, to some extent, but also the age problem.

      • Phillip Helbig Says:

        By 1990 it was firm that Omega was 0.3 or so. As far as I know, there was never any observational evidence which indicated a high value, and honest assessments came up with low values, even long before 1990. Why did people ever believe that Omega=1. There were primarily two reasons, which perhaps reinforced each other to some extent. One is the idea that inflation makes the universe flat, so that if one believes in inflation and if one hates the cosmological constant, then one is essentially left with the Einstein-de Sitter universe. The other was the preference for a simple universe.

      • telescoper Says:

        The main arguments for a high density Universe were (i) galaxy vs CMB dipoles and (ii) galaxy peculiar velocity studies. These persisted until well after 1990.

      • Phillip Helbig Says:

        Another reason might have been the misguided idea that one can avoid the flatness problem if Omega=1. That has nothing to do with inflation; if Omega=1 for any reason (and there is no cosmological constant), then it is always 1. That avoids the so-called instability problem (but not the fine-tuning problem). See the great article by Marc Holman for some insight into misguided ideas in connection with the flatness problem.

        Even if one is content that it solves only the instability problem, then it still wouldn’t work in a universe which is not exactly described by the Einstein-de Sitter model.

      • Phillip Helbig Says:

        The more one thinks about it, the more one finds even more arguments that the idea of the flatness problem is bogus.

      • Phillip Helbig Says:

        Although a survey of the literature, such as in Holman’s article, finds many arguments against the flatness problem, it is worth mentioning that even a quarter of a century ago there was by no means a consensus that the flatness problem is a problem. In fact, it is rather the reverse: it wasn’t really seen as a problem, and Guth actually added an appendix in his main inflation paper in order, as he explicitly writes, to convince the community that there is a problem (and hence that inflation can solve it), and only after that was it perceived by many to be a problem. As soon as that happened, several articles appeared pointing out why it is not a problem, but they have been ignored by most of the community and, crucially, by writers of textbooks, who tend to quote arguments from 40 years ago without even looking at the literature since then. (Imagine if one wrote a new textbook on observational cosmology and ignored all observations since 1980 or 1990!)

      • Phillip Helbig Says:

        Roberta Brawer’s MIT thesis, linked to above, is also available directly at MIT.

      • Phillip Helbig Says:

        “galaxy peculiar velocity studies. These persisted until well after 1990”

        But why did this “evidence” go away? IIRC, it wasn’t so much the observations but rather their interpretation via the POTENT method and so on. Even so, on balance observations indicated a low-density universe, even then.

      • telescoper Says:

        The evidence went away because the analyses were flawed. POTENT systematically overestimated Omega even in simulations, and the cosmological dipoles got a high mass density because they were assumed to converge long before they actually did.

      • Phillip Helbig Says:

        Yes, that jibes with my memory. Someone did a simulation with Omega=1, ran POTENT on it, and got Omega=1 back out. Then someone did a simulation with Omega=0.2 or whatever, ran POTENT on it, and also got Omega of around 1 back out.

      • telescoper Says:

        One of the problems with both of these methods is that it’s not a good idea to use something that depends only weakly on Omega as a means for estimating Omega. The dependence in both cases (assuming linear perturbations and no galaxy bias) is ~Omega^0.6

    • Phillip Helbig Says:

      Not Peebles, not then.

  2. I guess Van Waygaert represented this group, he was pretty much spot on !

  3. Phillip Helbig Says:

    “The picture was taken at a graduate school in cosmology in Leiden (in The Netherlands) in July 1995. In my memory that was a sweltering hot summer, which is my excuse for the informality of my attire.”

    Note that “leiden” means “suffer” in German.

  4. Phillip Helbig Says:

    “to embarrass all concerned”

    Not Rien. Interestingly, Rien has worked more on large-scale structure and so on than the determination of cosmological parameters.

  5. Regarding the so-called ‘flatness problem”, I think it’s fair to say that some theorists did find Dicke’s observation intriguing and somewhat puzzling, at least at the time. Some didn’t, but that doesn’t mean it wasn’t an issue for some

    • Phillip Helbig Says:

      It was definitely a huge issue for many. How often have I read that it’s one of the biggest unsolved problems in science or whatever! The problem now is that despite a score or so of papers—including one by Telescoper himself.—pointing out holes in the argument of Dicke and Peebles, none of which seems to have been rebutted, people go on repeating that argument as if no-one had thought about it in the last 40 years. That would be like writing a modern cosmology textbook and waffling on about whether the Hubble constant is 50 or 100, or whether the steady-state or the big-bang theory is correct, or whatever.

      And those are not papers by obscure people in obscure journals, but rather written by people such as Ehlers, Rindler, Kantowski, Lake, Gibbons, Hawking, etc. and published in MNRAS, PRD, PRL, CQG, GRG, FoP, etc,

      • If you mean papers like Coles and Ellis 1994, they certainly find holes in probability arguments and inflationary arguments, but don’t seem to give a mechanism for omega much less than one that overcomes Dicke’s runaway argument … it may be a matter of comprehension for us non-theorists

      • Phillip Helbig Says:

        There are (at least) two problems: the fine-tuning argument (why Omega was close to 1 at the beginning) and the instability or runaway argument (why is Omega still order of 1 today, even if we understand why it was very close to 1 early on). Most papers address the first problem. A very short summary is that Omega is close to 1 early on because that is what happens in non-empty big-bang F(L)RW models. Of course, one can ask why the Universe is (almost) F(L)RW, but that is a different question. The way it is usually phrased is that there is a problem even given the fact that the Universe is (almost) F(L)RW. (With the horizon or isotropy problem, on the other hand, asking why it exists is essentially equivalent to asking why FLRW.) Most of the papers attempt to quantify the idea in some way.

        With regard to the second problem, Lake showed that it doesn’t exist if there is a positive cosmological constant in a universe which expands forever (ours is an example). In fact, one needs fine-tuning in order to get, not to avoid, large values of Omega—a reverse of the usual argument. If the universe collapses in the future, then Omega becomes arbitrarily large, but only during the relatively short and special time near maximim expansion, so it is no surprise that a typical observer doesn’t measure large Omega.

        So, in summary, there is no flatness problem in classical cosmology. Remember that when it was formulated, Omega was known to be between 0.05 and 10 or whatever; not much precision. We now know that the Universe is very close to flat. In that case, the timescale argument probably can’t explain the observed degree of flatness, but that issue is rather moot since that is not our Universe. Whether Lake’s argument can depends on how much tuning is considered too fine.

        There are also weak-anthropic arguments which suggest that we shouldn’t observe a large value of Omega; that is what my latest paper is about (free (for me and for readers) access in MNRAS).

        So, the problem has been solved; the only flatness problem left is why that is not better known. 🙂

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