Undergrad Does The LCDM Model need reworking?

[Berenices]

Greetings,
I am slightly curious as to whether the LCDM model may require a slight tweaking. The reason I am curious is because I recall reading a while back that removing the homogeneity assumption from our current models could assist in accounting for some of the large-scale structures, and voids that we see in the universe. Have any of you ever discussed these speculations before?

The age of the universe is calculated by extrapolating the current rate of expansion backwards, and if the homogeneity assumption fails on some scales, would this mean that some areas of the universe would be older relative to others?

Sorry if I'm a little misguided.
Thanks in advance.

 

[jerromyjon]

would this mean that some areas of the universe would be older relative to others?

The entire universe inflated simultaneously in this model, I believe. The universe didn't expand from a single point, every point expanded.

 

[jerromyjon]

I am slightly curious as to whether the LCDM model may require a slight tweaking.

If it were perfect we wouldn't be looking to improve our understanding and models to complete the picture.

 

[Greg Bernhardt]

Paging @Jorrie

 

[Chronos]

Tweaking the cosmological principle does not enjoy much mainstream support in cosmology.

 

[Darryl]

I personally think that it needs more than a tweak, if some of the basic assumptions of the theory are wrong, that clearly puts the entire theory in grave doubt. There does seem to be a growing body of evidence that points in the direction that the LCDM model is simply wrong.

I actually hope that it is, (to me) it is so messy and ad hoc, that is probably is wrong. But proving existing theories wrong is the job of science, that is what scientists do!

(http://www.scilogs.com/the-dark-mat...ience-discussions-on-the-future-of-cosmology/)

"The work by David Wiltshire (his lecture notes) and Thomas Buchert already indicates that inhomogeneities could possibly make the Universe appear to an observer situated within such an underdensity as if it's expansion is accelerating, although in truth it is not"

"That is, the inhomogeneities appear to be of the correct magnitude to eliminate the need for Lambda, Lambda (dark energy) merely being an apparent effect mis-interpreted by the supernova type 1a data. The reason lies in that a distant object's observed redshift depends in reality on the exact paths the photons travel in a universe which consists of time-changing voids and over-densities, and this is a different redshift computed assuming a homogeneous and isotropic expanding Universe."

So if you take away from the LCDM the assumption that redshift is a measure of distance and expansion, where does that lead the LCDM model? That is the kind of issue that no amount of tweaking is going to fix.

 

[Berenices]

Surely all the evidence does point to an accelerating expansion? However I think these propositions probably need to be at least tested to see if they fit the universe.

 

[Berenices]

Okay, Thomas Buchert says that the results are intermediate. Removing the cosmological principle could explain dark energy, but the effects could also be not very pronounced. He does say that however pronounced they are it will probably have an effect on the modelling of the late universe. IMO the effects could not be so pronounced as to remove the need for the cosmological constant, but It will be interested to see where this goes.

 

[phyzguy]

Greetings,
I am slightly curious as to whether the LCDM model may require a slight tweaking. The reason I am curious is because I recall reading a while back that removing the homogeneity assumption from our current models could assist in accounting for some of the large-scale structures, and voids that we see in the universe.

Why do you think the existence of large-scale structure is inconsistent with LCDM? Do you have any evidence or references? There have been many studies, for example this paper, that show that measurements of large-scale structure are very consistent with the LCDM model.

 

[Berenices]

To my knowledge, The Great Cold spot is an anomaly that has a 1% chance of forming due to random fluctuations, that in itself does not warrant that the LCDM model is wrong. However, we also see structures - such as the Hercules-Corona Borealis Great Wall, which should not form within 13 billion years according to the LCDM model. While I agree that the LCDM model does an excellent job modelling the universe as a whole (at least in it's present epoch) I would like to know whether tweaking it could account for these above stated inconsistencies.

 

[MarkJW]

Tweaking the cosmological principle does not enjoy much mainstream support in cosmology.

You should watch the movie The Principle. It discusses the issue of the cosmological/Copernican Principle, and features Lawrence Krauss, Max Tegmark, Michio Kaku, George FR Ellis, Julian Barbour, Bernard Carr and others (narrated by Kate Mulgrew, aka Captain Janeway of Star Trek). And yes it is quite controversial, and no, the scientists were not tricked (though initially Lawrence Krauss and Kate Mulgrew claim so, and others followed).

 

[Chronos]

The large scale structure age problem has been bandied about as evidence against the big bang since before Arp was 'Seeing Red' despite repeated refutations [re: http://www.astro.ucla.edu/~wright/lerner_errors.html] . It's like a zombie - it's a beast that just won't stay dead.

 

[phyzguy]

To my knowledge, The Great Cold spot is an anomaly that has a 1% chance of forming due to random fluctuations, that in itself does not warrant that the LCDM model is wrong. However, we also see structures - such as the Hercules-Corona Borealis Great Wall, which should not form within 13 billion years according to the LCDM model. While I agree that the LCDM model does an excellent job modelling the universe as a whole (at least in it's present epoch) I would like to know whether tweaking it could account for these above stated inconsistencies.

You keep saying that these large structures are not consistent with LCDM, but you haven't given me any references to support what you are saying. Here is another study looking explicitly at your question, whether the largest structures are consistent with LCDM, and their conclusion is that the largest structures are perfectly consistent with LCDM.

Before deciding to tweak LCDM, you have to show that there is a discrepancy that justifies the tweaking.

 

[Berenices]

Okay, AFAIK, that has to do with the SGW. The Hercules-Corona Borealis Wall is about much larger, is that type of discrepancy covered? Sorry, I am not fully informed about the refutation of how these large structures can form within our current framework. What I think matters is if we can prove that removing the Cosmological Principle describes the universe better or produces a different result. Then we can know if there may be something wrong with our models, is that not science?

 

[Chronos]

For a little discussion on the movie 'The Principle' see http://www.math.columbia.edu/~woit/wordpress/?p=6624

 

[phyzguy]

Okay, AFAIK, that has to do with the SGW. The Hercules-Corona Borealis Wall is about 2.5x larger, is that type of discrepancy covered? Sorry, I am not fully informed about the refutation of how these large structures can form within our current framework. What I think matters is if we can prove that removing the Cosmological Principle describes the universe better or produces a different result. Then we can know if there may be something wrong with our models, is that not science?

Why don't you try reading the paper and see if they discuss larger structures than the SGW?

 

[Berenices]

Okay, I will when I have time.
Note: about the referencing, of course fewer papers are going to be written about discrepancies from the LCDM. That is out of the domain of mainstream science. As stated, I think we need to at least put in the effort to see whether tweaking the Cosmological Principle produces different results or better describes the universe. Unless the results produced are exactly the same as with the Cosmological Principle, we will know there is room to improve the LCDM model.

 

[phyzguy]

Note: about the referencing, of course fewer papers are going to be written about discrepancies from the LCDM. That is out of the domain of mainstream science.

I don't know why you think this is true. Ask yourself, if I'm a researcher, especially a young researcher, which of the following would do more to enhance my career:

(A) Writing yet another paper showing that the LCDM model is a good description of the universe.

(B) Writing a paper showing a flaw in the LCDM model, and proposing an enhancement which fits the data better.

The answer is clearly (B). For this reason, many people are working trying to determine the limits of the LCDM model.

As stated, I think we need to at least put in the effort to see whether tweaking the Cosmological Principle produces different results or better describes the universe. Unless the results produced are exactly the same as with the Cosmological Principle, we will know there is room to improve the LCDM model.

OK, go for it.

 

[Berenices]

I think this is true due to the fact that we are educated and trained to think in a certain way, and this generally can lead to biases. I don't want to sound like a crank, but imagination is scorned within science, it seems we need to be logical and forward thinking, and adhere to the mainstream. Especially with something like modelling, the opposite should be the case. Simply apply Occam's Razor to this problem it would seem the most logical route would be to abolish the cosmological Principle and simply see what should happen. Do you disagree with this?

 

[phyzguy]

I think this is true due to the fact that we are educated and trained to think in a certain way, and this generally can lead to biases. I don't want to sound like a crank, but imagination is scorned within science, it seems we need to be logical and forward thinking, and adhere to the mainstream. Especially with something like modelling, the opposite should be the case. Simply apply Occam's Razor to this problem it would seem the most logical route would be to abolish the cosmological Principle and simply see what should happen. Do you disagree with this?

I think you are free to explore the consequences of any set of assumptions you'd like. Please go ahead and do the study you are proposing. I look forward to reading the paper.

 

[PeterDonis]

Any discussion of cosmological models, LCDM or otherwise, needs to be based on specific references. Vague gestures in the direction of an imagined alternative are not sufficient. Thread closed.