This model assumes the cosmological principle. The LCDM universe is homogeneous and isotropic. Time dilation and redshift z are attributed to a Doppler-like shift in electromagnetic radiation as it travels across expanding space. This model assumes a nearly "flat" spatial geometry. Light traveling in this expanding model moves along null geodesics. Light waves are 'stretched' by the expansion of space as a function of time. The expansion is accelerating due to a vacuum energy or dark energy inherent in empty space. Approximately 73% of the energy density of the present universe is estimated to be dark energy. In addition, a dark matter component is currently estimated to constitute about 23% of the mass-energy density of the universe. The 5% remainder comprises all the matter and energy observed as subatomic particles, chemical elements and electromagnetic radiation; the material of which gas, dust, rocks, planets, stars, galaxies, etc., are made. This model includes a single originating big bang event, or initial singularity, which constitutes an abrupt appearance of expanding space containing radiation. This event was immediately followed by an exponential expansion of space (inflation).

Astronomy Without A Telescope – Assumptions

17 Apr , 2011

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The current standard model of the universe, Lambda-Cold Dark Matter, assumes that the universe is expanding in accordance with the geometrical term Lambda – which represents the cosmological constant used in Einstein’s general relativity. Lambda might be assumed to represent dark energy, a mysterious force driving what we now know to be an accelerating expansion of space-time. Cold dark matter is then assumed to be the scaffolding that underlies the distribution of visible matter at a large scale across the universe.

But to make any reasonable attempt at modelling how the universe is – and how it unfolded in the past and will unfold in the future – we first have to assume that it is roughly the same everywhere.

This is sometimes called the Cosmological Principle which states that when viewed on a sufficiently large scale, the properties of the Universe are the same for all observers. This captures two concepts – that of isotropy, which means that the universe looks roughly the same anywhere you (that is you) look – and homogeneity, which means the properties of the universe look roughly the same for any observers anywhere they are and wherever they look. Homogeneity is not something we can expect to ever confirm by observation – so we must assume that the part of the universe we can directly observe is a fair and representative sample of the rest of the universe.

An assessment of isotropy is at least theoretically possible down our past light-cone. In other words, we look out into the universe and receive historical information about how it behaved in the past. We then assume that those parts of the universe we can observe have continued to behave in a consistent and predictable manner up until the present – even though we can’t confirm whether this is true until more time has passed. But anything outside our light cone is not something we can expect to ever know about and hence we can only ever assume the universe is homogenous throughout.

You occupy a position in space-time from which a proportion of the universe can be observed in your past light cone. You can also shine a torch beam forwards towards a proportion of the future universe - knowing that one day that light beam can reach an object that lies in your future light cone. However, you can never know about anything happening right now at a distant position in space - because it lies on the 'hypersurface of the present'. Credit: Aainsqatsi.

Maartens has a go a developing at developing an argument as to why it might be reasonable for us to assume that the universe is homogenous. Essentially, if the universe we can observe shows a consistent level of isotropy over time, this strongly suggests that our bit of the universe has unfolded in a manner consistent with it being a part of a homogenous universe.

The isotropy of the observable universe can be strongly implied if you look out in any direction and find:
• consistent matter distribution;
• consistent bulk velocities of galaxies and galactic clusters moving away from us via universal expansion.
• consistent measurements of angular diameter distance (where objects of the same absolute size look smaller at a greater distance – until a distance of redshift 1.5, when they start looking larger – see here); and
• consistent gravitational lensing by large scale objects like galactic clusters.

These observations support the assumption that both matter distribution and the underlying space-time geometry of the observable universe is isotropic. If this isotropy is true for all observers then the universe is consistent with the Friedmann–Lemaître–Robertson–Walker (FLRW) metric. This would mean it is homogenous, isotropic and connected – so you can travel anywhere (simply connected) – or it might have wormholes (multiply connected) so not only can you travel anywhere, but there are short cuts.

That the observable universe has always been isotropic – and is likely to continue being so into the future – is strongly supported by observations of the cosmic microwave background, which is isotropic down to a fine scale. If this same isotropy is visible to all observers – then it is likely that the universe has, is and will always be homogenous as well.

Finally, Maartens appeals to the Copernican Principle – which says that not only are we not the center of the universe, but our position is largely arbitrary. In other words, the part of the universe we can observe may well be a fair and representative sample of the wider universe.

Further reading: Maartens Is the universe homogenous?



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The Eclectic Exterminator of Stupid Electricians
Member
The Eclectic Exterminator of Stupid Electricians
April 17, 2011 8:29 AM

Oh dear. You didn’t mention plasma cosmology in this article.
That is almost certainly going to turn the comments here into a free-for-all!

(Could you please be far more unmerciful to comments away from the subject and hand? Some will struggle with all the concepts here, and the usual pitch battle will immediately kill any interest in your story. Ta!)

Uncle Fred
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Uncle Fred
April 17, 2011 8:47 AM

I also wish that the UT writers removed EU/PC/God pet theories.

The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
April 17, 2011 8:55 AM
I personally like the dual light curves (double the second graphic). Such a figure shows not one but two different points in spacetime, where the representation of hypersurface has two past and future cones that eventually intersecting each other. It usefully explains the historical aspect of astronomy observations, where all things in the universe are observed in the historical past and not the present. The single past and future light cones are very difficult to understand this for novices. (In teaching I often give a verbal explanation, an have used the double past and future cones to describe these graphics; Using an observer on Earth and Alpha Centauri. It nicely ties space and time into something readily tangible… Read more »
damian
Member
April 17, 2011 2:30 PM
Cynical Indeed HSBC, Your the one trolling the comments on this site, indeed one of the three most prolific posters. ?? Hmmm. Lets not forget that your viewpoint has a tremendously strong Bias. Have you thought of starting your own Blog ? That way you could control who comments and be happy. BTW, Good work Mr Nerlich. I very much look forward to your posts each Sunday. Dare I say it, because it often encourages people with alternative viewpoints to comment. Even if the science behind them may be flawed I find it interesting and entertaining. Isotropic connected wormholes; nice. How do we go about detecting such things? Seems the only observed candidates might be singularities? Although I… Read more »
Lawrence B. Crowell
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Lawrence B. Crowell
April 17, 2011 2:42 PM
The article is a reasonable summary of Maartens paper. There are a couple of minor points of clarification. The light cone displayed here is on a local region of spacetime, where spacetime is flat. In the case of flat spacetime, anything which is outside our past light cone is not observable at the present time, though it might be in the past light cone in the future — a sort of physics version of future perfect. The hyperspace of the present is not a fixed space. What is fixed is the light cone, which is a congruency of light paths (null rays), while the hyperspace plane and the time axis can change for another observer on another frame.… Read more »
Richard Kirk
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Richard Kirk
April 17, 2011 4:49 PM
If you don’t know about the Oklo natural fission reactor, then it is worth looking up, because it is a really neat story, and it does not use telescopes. Briefly: a natural separation of uranium ores in a river bed made a natural fission reactor in what is now Gabon (Africa) about two billion years ago. As many of the fission products were trapped, reaction products were trapped, we are able to get a good measure of the ratios of the fission products. A samarium isotope allowed us to get a particularly good handle on the fine structure constant two billion years ago, and say with confidence that it was not significantly different to the value it is… Read more »
Lawrence B. Crowell
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Lawrence B. Crowell
April 17, 2011 8:11 PM
Claims about the variability of the fine structure constant tend to be a physics version of a “UFO.” Last year there was a claim that the fine structure constant changed with location, and there have been prior claims of variability in the fine structure, but these tend to go nowhere. The data tends to be at a 3-sigma range, so it is not usually very good. The nuclear reaction which took place in Gabon happened at a time the percentage of U235 was much higher than it is today, which is about .7%. U238 will not fission, and is used to produce plutonium. U235 decays, which is why there is so little of it around. It has been… Read more »
IVAN3MAN_AT_LARGE
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IVAN3MAN_AT_LARGE
April 18, 2011 4:07 AM

Actually, U-238 is fissionable but not fissile: it undergoes induced fission when subjected to an intense flux of very energetic neutrons with over 1 MeV of kinetic energy per neutron; however, too few of the neutrons produced by U-238 fission are energetic enough to sustain a chain reaction in U-238.

(I looked that up in Wikipedia; I did not pull it out of my ass like those EU/PC guys do!)

IVAN3MAN_AT_LARGE
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IVAN3MAN_AT_LARGE
April 18, 2011 4:19 AM

(P.S. Out of theirs, that is.)

IVAN3MAN_AT_LARGE
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IVAN3MAN_AT_LARGE
April 18, 2011 4:26 AM

(P.P.S. Oh, er… pardon my French, but I’ve had too much beer tonight!)

Question
Member
Question
April 18, 2011 8:26 AM

thanks for mentioning that. i was wondering about that too but had just assumed that i was the one who was wrong.

Nancy Atkinson
Admin
April 17, 2011 7:35 PM

HSBC- if you would quit goading, encouraging and replying to the EU folks, they might stop posting. But no, first thing you do on this article is post something encouraging them, saying that it is inevitable for them to post here. Just stop it. You’re getting worse than they are. Plus your comments are getting way too long (not on this one, but I’ve seen some of your “books” on other articles). I’ll start removing them if you don’t shorten up your comments.

The Eclectic Exterminator of Stupid Electricians
Member
The Eclectic Exterminator of Stupid Electricians
April 17, 2011 11:56 PM

Thank you for publicly embarrassing me. I really do appreciate it.

If you think my first or other comments are a problem or breaks the rules, then delete it.

Also there is a great difference between length and relevance. If it is irrelevant, then delete what you must. I.e. My long comment on Astronomy Without A Telescope – Forbidden Planets” by Steve Nerlich on December 11, 2010 (I’d assume you’ll wanna delete that too?)

Now do I behave like a good boy and get back it my box?
Well I’ll leave that to you.

The Eclectic Exterminator of Stupid Electricians
Member
The Eclectic Exterminator of Stupid Electricians
April 18, 2011 12:11 AM

Actually, as this site uses WordPress, you can use a Comment Length Limiter plug-in that uses a character count. (300 words would suffice.)

Also, as requested before, could you make the “Recent Comments” extended from 5 to 10 or 15. It would make it easier to respond to comments without searching through the stories. (We’ve had no response to this request even though it has been asked several times by me and others.)

Thank you.

Question
Member
Question
April 18, 2011 12:19 AM

… a nicely balanced article mr. nerlich. you have been informative but also mindful of inserting at least a couple disclaimers which serve to satisfy readers such as myself.

ex: “…we must assume that the part of the universe we can directly observe is a fair and representative sample of the rest of the universe…”

regardless of the evidence to support this statement, the fact is that we’ll most likely never know for sure if it’s true or not. overcoming these types of uncertainties (and moving forward) is a key point in the progression of the individual in his/her acceptance of the limitations of humanity’s scientific potential.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
April 18, 2011 11:41 AM
Maartens do valuable work in teasing apart what theory and observational dependencies there are, and how to tackle them. I’m reminded on how I saw something similar recently on exoplanet scientist Sara Seager’s web page. She has looked on how transit light curves alone can’t be used to ascertain structure of the planets. But I believe the Kepler-11 system showed one way such results may eventually break down, as the near planet’s tugging revealed individual mass. Btw, I don’t really like using the term “assumption” here, as it suggests a relation that isn’t exactly true. Models rely on assumptions to make predictions. One can abstract that to axiomatic theory axioms, bearing in mind that very little modern physics… Read more »
EdR
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EdR
April 18, 2011 1:39 PM

As a regular reader of UT stories, an occasional reader of commentaries, and a rare poster, I agree with HSBC that postings by the lunatic fringe are annoying and generally useless. It probably would take too much time and energy (and add fuel to the fire), but I’d like to see obvious LF postings severely truncated and the remainder of the messages linked to another location where those interested can follow them, but those of us who find them tedious and annoying don’t have to slog through them to find real science. Too much to ask, I’m sure, but it feels better saying it!

interI0per
Member
interI0per
April 18, 2011 5:20 PM

as an aside to all the interesting comments, it strikes me how useful a visual representation can be.
these days with 3d animation we can convey complex concepts with fluid ease.
a meta-language that invites more people to speculate.
then tap out a reply with Morse code.

Surak
Member
Surak
April 18, 2011 8:22 PM
Great story Steve, I enjoy your work here, your website and 365 podcast contributions. I hadn’t thought about the assumptions in terms of the light-cone diagram before, it makes a lot of sense and helped me understand it better. I usually just breeze over HSBC and LB’s comments, they obviously know far more than I do and do good work to clarify things, but sometimes they’re more like professors tired of reading through another pile of undergrad reports. I appreciate your attempts to translate the journal papers into something I can read even if they’d rather pull it closer to the journal level now and then. I can’t stand the EU quacks either … but at least they… Read more »
The Eclectic Exterminator of Stupid Electricians
Member
The Eclectic Exterminator of Stupid Electricians
April 19, 2011 7:15 AM
Yet to the point. It is interesting to me that we must assume isotropy and homogeneity is the same everywhere in our universe. It is unprovable only because we cannot travel to where we can test it. This same point is central to EU/PC lot too, where they assume that electrical experiments made on Earth also must be both show both isotropy and homogeneity. We could accept that there is differences in the structure of the universe and how it behaves, then no current theory can be absolute or usable everywhere. In astrophysics it is how constant is gravitation, and if it were different elsewhere, it would change the nature of objects. I.e. densities, and there for their… Read more »
The Eclectic Exterminator of Stupid Electricians
Member
The Eclectic Exterminator of Stupid Electricians
April 19, 2011 7:18 AM

Has anyone noticed that the “EU guys” haven’t shown up here?

Why is that?

DrFlimmer
Member
DrFlimmer
April 19, 2011 9:11 AM

Not to complain, but wouldn’t a silent joy about this fact be enough?
I think, we should agree, that we do not mention them (yeah, I know, it’s tempting…) as long as they are not here.

DrFlimmer
Member
DrFlimmer
April 19, 2011 9:12 AM

P.S.: Just like in Quantum Mechanics: As long as you don’t observe something, it’s neither there nor happening! grin

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