Archive for standard cosmological model

Challenges for the Standard Cosmological Model

Posted in The Universe and Stuff with tags , , , on June 14, 2021 by telescoper

I recently came across a comprehensive review article on the arXiv and thought some of my regular readers might find it interesting as a description of the current state of play in cosmology. The paper is called Challenges for ΛCDM: An update and is written by Leandros Perivolaropoulos and Foteini Skara.

Here is the abstract:

A number of challenges of the standard ΛCDM model has been emerging during the past few years as the accuracy of cosmological observations improves. In this review we discuss in a unified manner many existing signals in cosmological and astrophysical data that appear to be in some tension (2σ or larger) with the standard ΛCDM model as defined by the Planck18 parameter values. In addition to the major well studied 5σ challenge of ΛCDM (the Hubble H0 crisis) and other well known tensions (the growth tension and the lensing amplitude AL anomaly), we discuss a wide range of other less discussed less-standard signals which appear at a lower statistical significance level than the H0 tension (also known as ‘curiosities’ in the data) which may also constitute hints towards new physics. For example such signals include cosmic dipoles (the fine structure constant α, velocity and quasar dipoles), CMB asymmetries, BAO Lyα tension, age of the Universe issues, the Lithium problem, small scale curiosities like the core-cusp and missing satellite problems, quasars Hubble diagram, oscillating short range gravity signals etc. The goal of this pedagogical review is to collectively present the current status of these signals and their level of significance, with emphasis to the Hubble crisis and refer to recent resources where more details can be found for each signal. We also briefly discuss possible theoretical approaches that can potentially explain the non-standard nature of some of these signals.

Among the useful things in it you will find this summary of the current ‘tension’ over the Hubble constant that I’ve posted about numerous times (e.g. here):

What is the Standard Cosmological Model?

Posted in The Universe and Stuff with tags , , , , on May 12, 2021 by telescoper

There’s a nice little paper – a summary of a talk – by Eric Linder on the arXiv here. The abstract is:

Reports of “cosmology in crisis” are in vogue, but as Mark Twain said, “the report of my death was an exaggeration”. We explore what we might actually mean by the standard cosmological model, how tensions – or their apparent resolutions – might arise from too narrow a view, and why looking at the big picture is so essential. This is based on the seminar “All Cosmology, All the Time”.

You can find a PDF here.

The paper discusses not only the question “what is the standard cosmological model?” in a fairly general way but also the more basic question “what is cosmology?” I’d think you’d be surprised how different would be the answers to that question from different cosmologists!

Anyway, I usually say when I give talks that the following are the six main ingredients of the standard model:

  1. General Relativity
  2. The Cosmological Principle
  3. Cold Dark Matter
  4. Cosmological Constant
  5. Primordial (nearly) Gaussian adiabatic fluctuations
  6. Inflation in the very early Universe

There are other ingredients of course, such as baryons and neutrinos but I don’t include them in a model because I feel one should distinguish at some level between the ingredients of a model and the  ingredients of the actual Universe. What I mean by that is that we know baryons exist (though we may not know their cosmic abundance precisely) but we don’t know for sure whether Cold Dark Matter exists.

Note that (1) isn’t really a model because it has no free parameters: it’s only when you add (2) that it comes a FLRW model with e.g. the curvature as a free parameter, and some assumed form of the energy-momentum tensor. The other ingredients have one or more free parameters, e.g. the density of CDM (3) and the value of Λ (4); these can be made more flexible by including, e.g., a dark energy term with equation of state w. Ingredient (5) needs the user to specify an initial power spectrum, which is at least two parameters (amplitude and slope), which may or may not be motivated by (6).

Anyway, the following items in the above list are – to a greater or lesser extent – open to question:

  1. General Relativity
  2. The Cosmological Principle
  3. Cold Dark Matter
  4. Cosmological Constant
  5. Primordial (nearly) Gaussian adiabatic fluctuations
  6. Inflation in the very early Universe

We’d be unwise to question only, e.g., 3 or 4 while ignoring the possibility that we may be wrong about the others!

 

Finding the Lost Baryons

Posted in Astrohype, The Universe and Stuff with tags , , , , , on June 3, 2020 by telescoper

Taking a break from examination marking I thought I’d post a comment on a recent paper in Nature which you can find on the arXiv here; see also a report here.

The paper, entitled A census of baryons in the Universe from localized fast radio bursts, is an important one which does seem to resolve a longstanding question often called the missing baryon problem. In a nutshell, the problem is that the density of baryons suggested by cosmological considerations – specifically the element abundances produced by Big Bang nucleosynthesis and the cosmic microwave background (CMB) – was, until recently, rather higher than that which has been observed by astrophysical measurements; by `baryonic material’ I mean basically protons and neutrons (whether or not they are in atomic nuclei).

In the framework of the standard cosmological model, The density of baryonic matter (denoted `Ordinary Matter’ in the following figure) contributes only around 5% of the overall mass-energy budget of the Universe:

The first thing to stress is that this paper says nothing about the `Dark Matter’ which, according to the standard model, makes up about 27% of the pie and which cannot be in the form of baryons if the CMB and nucleosynthesis measurements are correct. If it were baryonic it would participate in nuclear reactions and mess up the light element abundances and also interact with photons in such a way as to change the fluctuation spectrum of the cosmic microwave background. Having said that, `dark’ is better adjective to use for hidden baryons than it is for non-baryonic matter, as baryons can absorb light. Non-baryonic matter isn’t really dark, it’s transparent because it doesn’t interact at all with electromagnetic radiation. We are however in the dark about it.

Note that the total density of dark + ordinary matter is about 32%, just what George Ellis and I concluded way back in 1994.

We can be much more certain about baryons actually existing than we can about dark matter because. For one thing, we are made of them. It has, however, been known for ages that the total density of directly visible baryons (ie those associated with stars and galaxies) is much lower than this figure, leading to the conclusion that some of the baryons predicted by cosmologists must be in some invisible form(s). Some, for example, is found by X-ray emissions in dense galaxy clusters, but this component is still inadequate to account for all the missing matter.

It has been suspected for some time that the hidden baryons probably inhabit a diffuse Warm-Hot Component of the Intergalactic Medium which, according to simulations of structure formation, traces its own form of the cosmic web we see in the distribution of galaxies:

The diffuse state and inhomogeneous nature of this intergalactic medium makes it difficult to detect, as explained in the abstract of the paper, but adding a relatively new technique involving fast radio bursts to probe the distribution of matter along the line of sight to the observer, it seems that it has now brought out into the open:

Now the inventory of observed baryons matches the 5% figure we cosmologists always knew it would be, and all is well with the world!

P. S. I was informed on Twitter after posting this that there was a paper on this topic in Nature a couple of years ago the last sentence of the abstract of which reads:

We conclude that the missing baryons have been found.

WMAP: The Last Judgement

Posted in The Universe and Stuff with tags , , , , , , on December 21, 2012 by telescoper

It seems the the Wilkinson Microwave Anisotropy Probe, or rather the estimable team of people working on it, have produced yet another set of maps and key results. I believe this will be the final release from WMAP. The paper is on the arXiv here and it represents a synthesis of no less than nine years of measurements of the cosmic microwave background radiation:

Here’s the abstract:

We present the final nine-year maps and basic results from the WMAP mission. We provide new nine-year full sky temperature maps that were processed to reduce the asymmetry of the effective beams. Temperature and polarization sky maps are examined to separate CMB anisotropy from foreground emission, and both types of signals are analyzed in detail. The WMAP mission has resulted in a highly constrained LCDM cosmological model with precise and accurate parameters in agreement with a host of other cosmological measurements. When WMAP data are combined with finer scale CMB, baryon acoustic oscillation, and Hubble constant measurements, we find that Big Bang nucleosynthesis is well supported and there is no compelling evidence for a non-standard number of neutrino species (3.26+/-0.35). The model fit also implies that the age of the universe is 13.772+/-0.059 Gyr, and the fit Hubble constant is H0 = 69.32+/-0.80 km/s/Mpc. Inflation is also supported: the fluctuations are adiabatic, with Gaussian random phases; the detection of a deviation of the scalar spectral index from unity reported earlier by WMAP now has high statistical significance (n_s = 0.9608+/-0.0080); and the universe is close to flat/Euclidean, Omega_k = -0.0027 (+0.0039/-0.0038). Overall, the WMAP mission has resulted in a reduction of the cosmological parameter volume by a factor of 68,000 for the standard six-parameter LCDM model, based on CMB data alone. For a model including tensors, the allowed seven-parameter volume has been reduced by a factor 117,000. Other cosmological observations are in accord with the CMB predictions, and the combined data reduces the cosmological parameter volume even further. With no significant anomalies and an adequate goodness-of-fit, the inflationary flat LCDM model and its precise and accurate parameters rooted in WMAP data stands as the standard model of cosmology.

The main reason for posting this is to acknowledge the remarkable impact WMAP has had on the field of cosmology. The standard model does indeed account for most available cosmological data extremely well. I’m not entirely sure about the “no significant anomalies” bit in the last sentence, in fact, but I won’t argue with it as it depends entirely upon what you mean by significant. It’s not exactly proven that the fluctuations have “random phases” either. We’ll just have to see whether data from Planck, due to be released next year, will reveal evidence of any physics beyond the standard framework WMAP did so much to establish.

Dark Energy is Real. Really?

Posted in Astrohype, The Universe and Stuff with tags , , , , , on May 20, 2011 by telescoper

I don’t have much time to post today after spending all morning in a meeting about Assuring a Quality Experience in the Graduate College and in between reading project reports this afternoon.

However, I couldn’t resist a quickie just to draw your attention to a cosmology story that’s made it into the mass media, e.g. BBC Science. This concerns the recent publication of a couple of papers from the WiggleZ Dark Energy Survey which has used the Anglo-Australian Telescope. You can read a nice description of what WiggleZ (pronounced “Wiggle-Zee”) is all about here, but in essence it involves making two different sorts of measurements of how galaxies cluster in order to constrain the Universe’s geometry and dynamics. The first method is the “wiggle” bit, in that it depends on the imprint of baryon acoustic oscillations in the power-spectrum of galaxy clustering. The other involves analysing the peculiar motions of the galaxies by measuring the distortion of the clustering pattern introduced seen in redshift space; redshifts are usually denoted z in cosmology so that accounts for the “zee”.

The paper describing the results from the former method can be found here, while the second technique is described there.

This survey has been a major effort by an extensive team of astronomers: it has involved spectroscopic measurements of almost a quarter of a million galaxies, spread over 1000 square degrees on the sky, and has taken almost five years to complete. The results are consistent with the standard ΛCDM cosmological model, and in particular with the existence of the  dark energy that this model implies, but which we don’t have a theoretical explanation for.

This is all excellent stuff and it obviously lends further observational support to the standard model. However, I’m not sure I agree with the headline of press release put out by the WiggleZ team  Dark Energy is Real. I certainly agree that dark energy is a plausible explanation for a host of relevant observations, but do we really know for sure that it is “real”? Can we really be sure that there is no other explanation?  Wiggle Z has certainly produced evidence that’s sufficient to rule out some alternative models, but that’s not the same as proof.  I worry when scientists speak like this, with what sounds like certainty, about things that are far from proven. Just because nobody has thought of an alternative explanation doesn’t mean that none exists.

The problem is that a press release entitled “dark energy is real” is much more likely to be picked up by a newspaper radio or TV editor than one that says “dark energy remains best explanation”….

Share/Bookmark