8 thoughts on “Weekend reflections

  1. Are there any physicists reading my favourite blog?

    The ANU’s Brian Schmidt (and 2 colleagues) have won a Nobel for their work demonstrating that the universe is not just expanding, but that the rate of expansion is accelerating – and I’ve got a question.

    It’s probably a stupid question and maybe the answer is obvious, but here goes:

    If space itself is expanding at an ever-increasing rate, is the expansion the same everywhere in the universe? That is, is it the same within galaxies as between galaxies?

    And is the same at different scales? That is, is it the same at the molecular scale as it is at the galactic scale?

    If it is the same everywhere and at the smallest and largest scales, wouldn’t that mean that the expansion wouldn’t really have a detectable effect, because everything is expanding together, including our Milky Way, our solar system, our planet, and us ourselves?

    Or does the gravity inherent in matter counteract the expansion locally, by holding everything together? And if this is the case, is our galaxy held together against or within an expanding matrix of space which becomes steadily more diffuse?

    Or does gravity, in holding together the matter of our galaxy, slow or neutralize the expansion of space in that vicinity? And if that is the case, doesn’t that suggest that gravity has a “grip” on space, has traction?

    I’d love to see what smarter people than me have to say about this.

  2. I am no physicist but Wikipedia says;

    “An expanding universe means that density drops due to continual space being added between all matter. If acceleration continues, eventually all galaxies beyond our own local supercluster will redshift so far that it will become hard to detect them, and the distant universe will turn dark.”

    At the sub-atomic level, I don’t think there is “space between matter” in the classical sense because neither classical space nor classical matter exist at the sub-atomic level. Therfore, it is a dubious question to me to suggest that sub-atomic “space” is expanding. But then I am not a physicist

    Your question “if it is the same everywhere and at the smallest and largest scales, wouldn’t that mean that the expansion wouldn’t really have a detectable effect? “, appears to answer itself because they have detected the expansion, therefore it must in essence be an expansion relative to sub-atomic phenomena OR (more significantly in this case) an expansion relative to the metric of the speed of light.

    The best way to envisage the expansion is to draw 10 c piece size circles on a half inflated balloon. Then inflate the balloon to full. The circles expand and the distance between the circles expands. If you are on the rim of circle (like being on the rim of our galaxy) you see the universe expand and measure it by the metric of light-years (which is equivalent to your original 10 c piece template being the metric for the balloon experiment).

    Current science suggest an early super-inflationary period, a slowdown and now a speeding up again of inflation. So gravity is losing against another, as yet, not-understood force. Nothing known can travel faster through space than the speed of light (except for recent detections of neutrinos*) but strangely enough space itself can expand much faster than the speed of light and did so in the early moments of the universe.

    * http://www.guardian.co.uk/science/2011/sep/22/faster-than-light-particles-neutrinos

  3. This explains it a bit better.

    “Metric expansion is a key feature of Big Bang cosmology and is modeled mathematically with the FLRW metric. This model is valid in the present era only at relatively large scales (roughly the scale of galactic superclusters and above). At smaller scales matter has clumped together under the influence of gravitational attraction and these clumps do not individually expand, though they continue to recede from one another. The expansion is due partly to inertia (that is, the matter in the universe is separating because it was separating in the past) and partly to the repulsive force of dark energy, which is of a hypothetical nature, but it may be the cosmological constant. Inertia dominated the expansion in the early universe, and according to the Lambda-CDM model (?CDM model) the cosmological constant will dominate in the future. In the present era they contribute in roughly equal proportions.”- Wikpedia

  4. @Ron E Joggles
    In answer to the other part of your question, “Is the universe the same everywhere?”

    The universe can only be observed within the “cone” of bounded by the speed light, going backwards in time, that is, you have to be able to see it. The other stuff outside this cone is assumed to be like what you can see but this is practically unknowable. We assume that the laws of physics are constant and the broad scale distribution of matter is roughly uniform from place to place. This is an assumption with some support, eg, the basic chemistry of the early earth a few billion years ago looks to be the same as now, and, the life cycle of stars appears to be about the same everywhere we can see both in space and time. Small differences in fundamental constants would have big effects on things like star evolution. On the other hand, there are some unresolved discrepancies with the uniformity assumption such as the “Australian Dipole”, an apparent variation in the fine structure constant. This causes very tiny variations in properties of stars in different directions.

    The acceleration is deduced from a statistical analysis of the spread of a large number of radio sources (galaxies) going back towards the big bang, which show that the expansion – looking from here and now – appears to have been slower in the past. No one knows why it might be happening and explanations like dark matter might (kinda) work mathematically but are unsatisfying because no one has ever observed the stuff, except tautologically via the observations that require something to be causing it.

    A further problem is that there is no generally accepted theory of the topology of space-time; there are different theories but no decisive evidence. The theories mostly assume constant physical constants. If the physical constants are not actually constant but vary across the universe, not only do we need new theories, but the whole thing is up for grabs, because a lot of observational results like the acceleration assume that physical constants are constant across time and space. This would be a weirder result than the acceleration – the space-time under your feet gets absolutely squishy.

    The expansion of the universe doesn’t have much local effect, the radius of a hydrogen atom, the earth, the solar system, and even the galaxy remain about the same – driven by local forces – but the galaxies get on average further away from each other. They also burn out. Gravity does counteract the expansion so things could slow down and fall back but there appears to be enough kinetic energy to overcome gravity so that expansion can continue indefinitely, albeit slowing slightly over time. The observed acceleration comes out of left field; it’s not at all clear what could be speeding the expansion up. Unless it is found to be some kind of artefact of the measurement process, which is very unlikely, it’s new territory.

  5. Thanks Ikonoclast and Jim Birch for informative responses. I also this morning heard Brian Schmidt on the Science Show (the repeat), and he reckons the accelerating expansion is between galaxies and not within them, in fact, as we’ve heard recently, our galaxy will eventually merge with a neighbour.

    Surely this suggests that the presence of matter slows the expansion of space locally, and that would imply that matter does have traction, or friction, on the stuff of space.

    To use Ikonoclast’s analogy of the half-inflated balloon and 10 cent coin-sized circles, if there were actual coins glued to the balloon, as it is inflated further the coins will prevent that portion of the balloon expanding.

    On the other hand, a coin just sitting on the balloon will not stretch (obviously) but the surface of the balloon beneath it will.

    So space itself is of a nature that is impacted by, affected by, matter, and presumably energy too. This seems to me (in my naivety) to be quite a profound idea.

    Space isn’t nothing!

  6. @Ron E Joggles

    “Space isn’t nothing!”

    Yes, I think that is quite true. It is what is called (in physics) a “field”. You would have heard of “gravitational field”, “electro-magnetic field” and so on. Space, or rather space-time is a field in this sense, as I understand it; a scalar field. Fields in physics are essentially geometrical-mathematical models used in physics to explain the properties of what we experience and detect as fields in empirical reality. Thus, a field as described in physics is a physical quantity associated to each point of spacetime.

    Arthur Schopenhauer wrote in §18 of On the Fourfold Root of the Principle of Sufficient Reason (1813): “…the representation of coexistence is impossible in Time alone; it depends, for its completion, upon the representation of Space; because, in mere Time, all things follow one another, and in mere Space all things are side by side; it is accordingly only by the combination of Time and Space that the representation of coexistence arises.”

    H.G. Wells wrote in a novel, “any real body must have extension in four directions: it must have Length, Breadth, Thickness, and Duration.”

    “Mathematically, a scalar field on a region U is a real or complex-valued function or distribution on U.[1][2] The region U may be a set in some Euclidean space, Minkowski space, or more generally a subset of a manifold, and it is typical in mathematics to impose further conditions on the field, such that it be continuous or often continuously differentiable to some order. A scalar field is a tensor field of order zero,[3] and the term “scalar field” may be used to distinguish a function of this kind with a more general tensor field, density, or differential form.” – Wikepedia

    I don’t fully understand the above quote by the way.

    Both philosophy and modern physiscs seem to confirm that nothing cannot exist. Therefore everything is something. Even the “empty” field of spacetime is something and has measureable properties and measurable interactions with mass/energy. This is as I understand it. I am no physicist.

    The gravitational and spacetime controversy has at least three possible “solutions” given in “Is Spacetime a field?” by Denis Lehmkuhl, Oxford University 2007 ;

    “So what exactly does it mean to say that GR (General Relativity) associates the geometry of spacetime with the gravitational ?eld? There are essentially three possibilities:

    1. Describing both gravity and (spacetime) geometry in terms of the metric tensor means
    eliminating the picture of gravity as a force (?eld): it is rather a manifestation
    of spacetime geometry. Thus, GR is the geometrization of gravity.

    2. The possibility of associating both geometry and gravity with the metric
    tensor shows that spacetime geometry is nothing more than the gravitational
    ?eld itself. It is not gravity that is a manifestation of the geometry of
    spacetime. Rather, the geometry of spacetime is a manifestation of the
    gravitational ?eld. Thus, GR is the ?eldization of geometry.

    3. We should take the fact that both gravity and geometry are described by
    the metric tensor to heart: neither the geometry-perspective (1.) nor the
    ?eld-perspective (2.) are privileged. Rather, GR can be seen as asserting
    the gravitational ?eld and the geometry of spacetime to be indistinguishable.”

    Lehmkuhl argues for the third position.

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