Astronomy Without A Telescope – Assumptions


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?

32 Replies to “Astronomy Without A Telescope – Assumptions”

  1. 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!)

      1. Huh? I exactly thought the same thing when I read your response too.
        So you aren’t aware of the continuous anti-science / pseudoscience comments among yours (and others) articles in Universe Today. More often than not comments in your “Astronomy Without A Telescope” series here are deliberately sidelined by alternative agendas. I.e. PC/EU (and other) and their stance that the universe is infinitely large and of considerable older age, the non-existence of black holes and the Big Bang, etc.
        I thought the ideas of these article are to disseminate current theory and ideas of understanding the cosmos. Often wee novices haven’t a ghost of chance in understanding your words in this article, as they are drowned out by the diversions of the avoiding naysayers!
        (Perhaps, after all this time, I thought you would have understood that? Perhaps me assuming this too, just shows I was wrong?)

      2. when i started reading UT all those years ago, i too was a wee novice. now i think i am simply a novice. i don’t have the education to grasp some of these concepts though i give it my all. and the math just leaves me dizzy. but i still love to read here and broaden my limited understanding of the universe. if only i had been as passionate about the learning when i was in school.
        one of the things that i enjoy most are the comments from other schools of thought. yes, they get out of hand from time to time. and the trolling can be tiresome when i’m not in the mood for it. but i do have to say that these alternate views, this psuedo science, has helped me to understand standard cosmology and big bang principles even more. anaconda, for all his rambling diatribe and caustic soapboxing, was passionate about his pet theory and inspired me to have a look. i looked and and i, an admitted wee amateur, came up with the conclusion that it is wrong. but in the process i learned more about the prevailing theories. my point being that without looking at EU and PC i would not appreciate the standard theory as well as i do. yes, many readers here will be sidetracked or discouraged and that is unfortunate. can’t please or help everyone. a great part of my learning process came from the comments of people like yourself. Hon. Salacious B. Crumb, Lawrence B. Crowell, Torbjorn Larsson OM, and Ivan3man. my teachers. i read the articles here and sometimes say, “what?”, and then i get to where you guys pick the story apart and lay it bare. often what you write goes over my head but still i love to read it. and when the psuedo science crowd comes in, you are there to save the day and pick that apart too. i know it can be tiresome for you and i’m sure you are just quite done with squashing the trolls. i get that. but again i say that much of what i learned here is because you did just that. pick them apart and squash them. i say let them post. you do a handy job of knocking them out with only a comment or two. simple, direct rebuttal and statement of good science. these non-science posters usually turn into trolls and it is apparent in their behavior and lack of proof that they are just that, trolls. we can choose not to read them at any point. but for new readers who don’t know these other theories or even the standard one then this is such good way to see the facts. HSBC, you are one of my favorite reads here. if i’m in a hurry in the mornings sometimes i just skip the story and go to comments to see if you think it’s worth reading. but you are becoming more vitriolic and abusive of late. i understand your’e frustrated. i know it’s the same thing over and over and over. but please don’t lose heart. your calm and rational replies and explanations have meant the world to me. deep breaths please. you can’t teach everyone at once and not everyone will be willing to be taught. but for those of us here wanting a clear picture, please keep a level head and keep the science coming. thank you.

      3. Thanks, I truly could not have said that better. I retired a year ago and this site is part of my personal retirement education plan, especially the comments. Admittedly, LC usually loses me in the math and HSBC can get out there sometimes, but the educational value is worth the time spent trying to wrap my head around it all.
        Nancy- Please don’t whup HSBC too hard. I and others would miss him a lot. Thanks.

  2. 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 without to much imagination. (The usual death nell of failing to understand the concept behind such graphics for novices, that often may lead to influence of pseudoscience individuals; who often have not understood it either.)

    Thanks for the article.

    1. No worries.

      My fail if the story is totally incomprehensible, but I figure anyone keen on reading cosmology articles is going to stumble upon a light cone eventually. It hadn’t occurred to me that presenting two intersecting cones would be more comprehensible, but I see your point.

      You folks seem to predict more pseudoscience comments than we really get. I think this advance negativity kills comment streams. Why not see if it happens first and then champion the science in response.

      1. There is an example on
        I.e. From J. Silk, The Big Bang, 2nd Ed.

        It is surprising to me how little fairly simple figures can be so confusing, and that using one of the main rules of teaching is to repetitive explaining the same thing but do so but repeating it from a couple of different angles.
        In regards cosmology “now” has different meaning in the universe. My “now” is different than some “now” distant object. I.e. If I die at this very instant, the universe will not know it until a message is received at that distance object as limited by the speed of light. Dual Minkowski spaces explain that very simply.

        (One of the big issue of confusion is what is outside the light-cone. Indeed, the y-axis in this instant is really like whole lot of frame express as a unit of time.)

        [Another advanced idea, is to reduce the unit of time to the length of the planck time, when the Minkowski light-cone actually brakes down at the quantum level. I.e. The counting of time is not invariant but is based on probability; so it is not a straight line/ surface as the edges of the cone.)

        As for your other point…

        Fair enough on the negative comments here. My words were certainly preemptive, and perhaps I’ve become too cynical to expect otherwise. I’d be happy to do what you suggest. Agreed. Let’s see what happens.

  3. 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 think any Sci-Fi leanings may be fanciful.

  4. 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. The time axis and the plane rotate towards each other, a bit like a lattice of boards, where one can collapse the lattice by squashing it together. This is a sort of visual version of special relativity.

    In a general spacetime with curvature these light cones exist in the spacetime with different orientations. One can then glue these together in a way and form a large scale “light cone” that has variable structure. In the case of inflation our large scale past light cone splays or fans outwards into the past to cover a very large spatial hyperplane. This is a manifestation of how inflation exponentially separated points of space at a furious rate, which also caused any local inhomogeneous bump or region to be stretched out to a near flatness situation. This manages to solve two problems. The first is the flatness problem, which is the question on how the conditions of the universe locally are the same as the most distant regions we observe. There must have been some causal connection between our local region and the most distant region we observe. However, that causal connection is something which we only observe in the present. Inflation permits the existence of a causal support in the very distant past between the most distant regions of the universe.

    This exponential stretching out of space, which is estimated to be 63 efolds, or e^{63} = 2.3 x10^{27}, which means that a region of space on the Planck scale ~ 10^{-33} cm gets stretched out to 10^{-6} cm (a micron) and the Hagedorn string scale 10^{-31}cm to about 10^{-4} cm, about the diameter of a hair. The initial universe tunneled out of some region, say due to a Dp-brane interaction or a vacuum region near a black hole singularity in some other region or cosmology. This initial region had an energy density of about 10^{100} times the current cosmological constant or Lambda, or was a region bounded by a horizon with a billion or so Planck units of area and a radius of about 10^{-26} cm. So inflation took this little region and blew it up into a 10cm region — about the size of a soccer ball. So the initial region of causal support was expanded in this way within about 10^{-25} seconds. In flat space light will travel d = ct = 3×10^10cm/sec x 10^{-25}sec = 3 x 10^{-15}cm — a much smaller distance. This is how inflation gives a causal support to the most distant regions of the observable universe.


  5. 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 today, and hence stuff then and stuff now is basically the same.

    These days, I understand the best figures on the stability of the fine structure constant come from looking at the spectra of quasars. One paper has argued that there has been a slight increase of about 5 parts per million in the fine structure constant over the last 10 billion years, and other don’t think it has changed by even that amount.

    1. 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 5 billion years or so since it was generated in a supernova.

      I agree with Nancy below. HSBC should hold his fire until after the EU guys show up.


      1. 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!)

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

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

  6. 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.

    1. 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.

    2. 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.

  7. … 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.

  8. 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 theory actually lend itself to such schemata. (It is powerful when it does, though.)

    But when you get away from ad hoc models and stray into theory land, the mutual dependencies of theory and observations, as well as between theories, makes the terminology less useful and more obstructing. The article list a whole lot of results that predicts the assumptions too.

    Also, when you test a theory on its predictions you test “assumptions” too. Are they useful, are there potential contender theories, are they even unique? In some cases you can probably say that the test certainty applies directly on those “assumptions”.

    [Speaking of which, I have to read Maartens, but I would guess that the FLRW metric is harder to get rid of than usually assumed. Such cosmologies, and such alone at a guess, results in zero energy universes.

    Without those you can’t have new universes tunneling out of old (zero energy means no third party required) or eternal inflation (zero energy means an infinite time system). And you want one of those pathways, or something closely related.]

    On the Minkowski light-cone something similar applies.

    It is fair enough that it, together with general relativity breaks down on quantum scales. Quantizied general relativity does when pushed to Planck scales; conversely you would like to have gravitons and what then of the field theory at such scales.

    Similarly semiclassical worldlines eventually breaks down in inflation when you follow them backwards. Blue-shifting means running up to such energies, and eventually worldlines must emanate from Planck lines. (In eternal inflation there is no upper limit on the whole set of worldlines though.)

    But that doesn’t mean that a Minkowski light-cone analog doesn’t exist on such scales in empty space away from extremal events. Which is why I look at those supernova timing events that promise to observe Lorentz invariance below Planck scale.

  9. 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!

  10. 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.

  11. 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 didn’t post the time-traveller youtube video story 🙂

  12. 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 appearance .
    [A more recent speculation is that if gravity were different (stronger) in the early universe, that it can drive stellar evolution at a more rapid rate, without the need for more massive PopIII star — the theoretical stars.]

    For electrical and magnetic fields it is [total or spectral] emissivity (ε) [The ability for a surface to emit energy by the amount of radiation.] It, too, is assumed to be constant.

    Another assumption is matter or antimatter. If there were regions of antimatter a visual observer could not tell the difference. The only way it would be possible are placed where they meet together, and this would be seen as a gamma ray source somewhere in the universe.

    What it comes down to difficult assumptions have to be taken. We have no choice), and this applied rationally and logically to many phenomena. As the universe seems to behave roughly the same way everywhere, by observation, this seems good enough to make astronomy and astrophysics viable topics of study.

    Gravity behaves simply, and the types of astronomical objects can be simulated, on all astronomical scales. Magnetic and electrical fields too can be simulated, but these have yet to be proven on all astronomical scales. (The assumptions are quite observationally different.) However, if either of these forces were proved to be inconstant, the the whole deck of cards would fall, (unless of course we could predicted the difference throughout the universe. Troublesome, because that could not be prove by experiment veery easily, if at all.

    (Thankfully by my pre-emption here, I am free of the distractions of others derailing the topic.)

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

    Why is that?

    1. 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.

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

Comments are closed.