Dark Energy: Part 2


Remember the basis for coming up with the idea of dark energy? It was the accelerating universe. And how did Perlmutter and Riess come to the conclusion that the expansion is accelerating? Physicist Sabine Hossenfelder explains.

There’s a certain class of supernovae, the type Ia, for which we know the energy emitted over time. The farther away it is, the dimmer is appears. Thus, from its brightness, one can infer the distance. In addition, the farther away it is, the longer it took to reach us. So indirectly, from its brightness, one can calculate how long back it occurred.

At the same time, one can determine its color. Just as the sound of a moving ambulance get shriller as it comes closer, the observed color of the supernova changes since it (and space itself) are moving away from us. The color can thus be used to infer the rate at which space is moving away.

Combine the above two points (time of the supernova v/s rate of expansion) for multiple supernovae at different times and you can calculate the change in the rate of expansion of the universe over time. And that is how we concluded, 20 years back, that we had a “runaway universe”, a universe whose rate of expansion was increasing over time.

Fast forward to present day. The data set of the supernovae 20 years back was small: just 110. Today, we have data from 1,000 supernovae. The new analysis used a bigger data set. More importantly, it factored in for two things: (1) the universe is expanding, (2) galaxies are moving too. Unlike Perlmutter and Riess, who hadn’t factored for the movement of galaxies. The surprising finding?
“What they found is that the best fit to the data is that the redshift of supernovae is not the same in all directions, but that it depends on the direction. This direction is aligned with the direction in which we move through the cosmic microwave background. And – most importantly – you do not need further redshift to explain the observations.”

Let’s rephrase that. If the latest study is right, it means the redshift can be accounted for by the movement of the galaxies alone. Put differently, it means the observed redshift doesn’t necessitate any additional contributions, say, by an expanding universe. To take that thought process to its logical conclusion: no additional contribution by the expansion of the universe -> no basis for believing the universe is expanding -> no need to postulate the existence of dark energy.

Wait a minute, didn’t we say in the last blog that there are other observations that point to the existence of dark energy?
“Well, all the other evidence for dark energy is not evidence for dark energy in particular, but for a certain combination of parameters in the concordance model of cosmology.”
Then again, this study in turn contradicts other data and analysis. Which means it’s not conclusive as a proof against dark energy, but it definitely means we need to dig in deeper and see what’s wrong or missing in all the analysis so far. Who knows what we’ll find?

Comments

  1. Ramiah KumarDecember 27, 2019 at 10:04 AM

    Sorry, this blog piece disappoints me!

    The preceding "Dark Energy: Part 2" gave me hope that I am going to get to know the subject better. What I find is that the science is diluted with simplistic presentation in this piece, through a layperson way of describing.

    Let me say what is my DISAPPOINTMENT in precise terms now: First the blog says “What they found is that the best fit to the data is that the redshift of supernovae ... this direction is aligned with the direction in which we move through the cosmic microwave background. And – most importantly – you do not need further redshift to explain the observations.” The parenthesis makes it clear that it is a quote from either a scientist or reliable science writer for laypersons' benefit. Correlating with the beginning of the blog, the writer is Physicist Sabine Hossenfelder, I am reasonably sure.

    And then the author's own summing up occurs: "Let’s rephrase that. If the latest study is right, it means the redshift can be accounted for by the movement of the galaxies alone. Put differently, it means the observed redshift doesn’t necessitate any additional contributions, say, by an expanding universe. To take that thought process to its logical conclusion: no additional contribution by the expansion of the universe -> no basis for believing the universe is expanding -> no need to postulate the existence of dark energy."

    There is a problem in this inferential statement. It seems to state that the the redshift of supernovas and the red shift of the galaxies otherwise measured in earlier to current times have the IDENTICAL story to tell - that "the universe is expanding". No anomaly at all!

    That the universe is expanding is no longer argued. It is true. The only point is the rate was faster during one period and the rate became slower and steady later. That anomaly OF FASTER EXPANSION earlier was explained away using a "first discorded then re-adopted constant" in the Theory of Relativity. For the later period, the constant was back to "its so called normalcy" that is '1'! So far so good. That's the same as what the blog writer concluded, REPEAT, "no additional contribution by the expansion of the universe -> no basis for believing the universe is expanding -> no need to postulate the existence of dark energy".

    What is NOT explained properly is, "What is the PRECISE reason for the astrophysics still needing dark energy to explain away the 'anomalous' expansion observed today?" Stated otherwise, "What exactly is that observation/detail of ANOMALY?"
    ----------

    The blog author has taken refuge to this pointed question by quoting, "“Well, all the other evidence for dark energy is not evidence for dark energy in particular, but for a certain combination of parameters in the concordance model of cosmology.”

    My precise disappointment is this: I read this blog eagerly only to be told "BUT FOR A CERTAIN COMBINATION OF PARAMETERS..." which leads to the question, "To be << in the concordance model of cosmology (verbatim quote) >> we very much NEED dark energy. Well, well - I am in the DARK (unless I have messed up understanding this blog), like dear DARK energy!!!!!

    Something (God's hand?) pushes the galaxies in an anomalous way right now, no? Why else you need extra energy?

    So, I eagerly await Dark Energy: Part 3! :-)

    ReplyDelete

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