Is the universe really expanding?

  1. Perhaps virtually 100% of physicists out there believe that we live in an 'accelerating universe', but as a person who's been researching the topic for a while, it seems to me the only evidence we have is based around redshift.

    I know redshift is a good enough argument to convince even Einstein, a radical thinker that the universe isn't static but expanding... but hey, what if there is another plausible explanation for redshift?

    I've been reading up on gravitational redshift, and could that be the cause of the redshift we see in absorption spectra of distant stars? Gravitational redshift is caused by time dilation, which would result in similar observations to what we see in absorption lines...

    A Google search yielded me this link with another possible explanation:

    It's on the Nature website so it's no joke!.

    Though the paper the article is based around isn't peer-reviewed, CTRL-F the name "Aranoff"
    and you'll find a comment by the person describing a similar peer-reviewed study.

    So, what do y'all think?
     
  2. jcsd
  3. Drakkith

    Staff: Mentor

    There are several plausible explanations, one in particular being tired light. However, every explanation other than redshift due to expansion requires even more alterations to known physics. Currently, expansion as the mechanism for redshift simply works better with fewer assumptions than anything else.

    No, as once the light would only be redshifted by the star's own gravity, which isn't nearly enough to cause the redshift we observe. On a large scale, gravity tends to cancel itself out when it comes to redshift. By that I mean that a ray of light traveling billions of light years experiences approximately equal pull from gravity in all directions so there is no redshift.

    To quote the article:

    It's an untestable interpretation, so it can't be given any weight as an explanation.
     
  4. Chronos

    Chronos 10,124
    Science Advisor
    Gold Member

  5. Thanks Drakkith, your answer is quite insightful.
     
  6. How can we know that the universe is expanding or it's just some kind of effect? There is this test called "Tolman surface brightness"

    "In a simple (static and flat) universe, the light received from an object drops inversely with the square of its distance, but the apparent area of the object also drops inversely with the square of the distance, so the surface brightness would be independent of the distance. In an expanding universe, however, there are two effects that reduce the power detected coming from distant objects. First, the rate at which photons are received is reduced because each photon has to travel a little farther than the one before. Second, the energy of each photon observed is reduced by the redshift. At the same time, distant objects appear larger than they really are because the photons observed were emitted at a time when the object was closer. Adding these effects together, the surface brightness in a simple expanding universe (flat geometry and uniform expansion over the range of redshifts observed) should decrease with the fourth power of (1+z)."

    Such simple test can discard most of alternative theories, including tired light or gravitational redshift. I must say the results of such test is quite interesting. Half of works favors expansion and half favors non-exansion.
     
  7. That's quite interesting, definitely gonna read up on it
     
  8. indeed, it is really hard to believe that the universe is expanding. It is even harder to believe that some galaxies are travelling faster than light.

    I'm sure physicists have considered the gravitational redshift in their calculations and redshift can be a pretty reliable source of information. But who can be sure?
     
  9. phinds

    phinds 9,146
    Gold Member

    There ARE no "galaxies are travelling faster than light", only galaxies that are RECEDING faster than light. In this as in other posts, you seem to believe that standard terminology is either not something you want to bother to learn, or is something that you just don't need to use. That attitude will not stand you well on this forum.
     
  10. It's space expanding faster than light. Space can expand faster than light.
     
  11. Drakkith

    Staff: Mentor

    It's important to understand that expansion is a rate, not a speed. We don't measure expansion by a speed, we measure it by how long it takes for the distance between objects to increase by a certain percentage, say 100%. Let's say it takes 1 billion years for expansion to double the distance between all unbound objects. If we measure the distance between us and any object, then wait 1 billion years, we will find that whatever that distance was, it is now doubled. This leads to an increasing recession velocity the further away an object is from us. Galaxies closer to us recede much more slowly than those far in the distance. Currently expansion causes a recession velocity of about 70 km/s per megaparsec. Which means that for every 3 million lightyears an object is from us its recession velocity will increase by 70 km/s. So at 30 million lightyears a galaxy should have a recession velocity of about 700 km/s. At 300 million lightyears it's 7,000 km/s. Etc, etc. Objects far enough away are thus receding at velocities greater than the speed of light. Which is okay. There is no global speed limit in general relativity like there is in special relativity.
     
  12. I find it hard to believe that the universe should go to the bother of existing at all, lol

    Mainstream science tends to go for the 'model that makes most sense out of most things'

    When the model gets badly broken it gets changed. When the main model becomes unusable for certain things then we might use another 'incomplete model' to paper over an inconvenient crack in a particular hole in a particular wall we are trying to plug.

    For every discrete phenomena one can find a dozen explanations but most of those explanations start to break down when you use them as a basis for a wider understanding. Tired light sounds like a good idea but AFAIK its not such good an idea when you take it for a long walk as it tends to get bogged down in the wider reality...a bit like a brick with no legs.

    http://en.wikipedia.org/wiki/Tired_light#Specific_falsified_models

    I think it was Fred Hoyle ( of steady state fame ) who coined the term Big Bang (a derisory term)

    However its not very often that you lose the argument but win the 'naming rights'


    Good rule of thumb

    Spend 80% of your time learning 'mainstream science' and 20% of your time chasing 'solid, plausible fairies'

    That way you include some history, some intriguing outliers and maintain a rust free intellectual sword.

    Some of those fairies might have legs as well as wings :-p ....i like to fondle the shiny wrapping paper before x-mas arrives.
     
    Last edited: Nov 25, 2013
  13. Chronos

    Chronos 10,124
    Science Advisor
    Gold Member

    That 'intellectual sword', as you put it, is constantly polished by science, not romantic idealism. Discredited theories earn their stature because they DO NOT survive in the face of observational evidence. It has nothing to do with history, or outliers, its all about EVIDENCE. Anything less is not science.
     
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