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Starts With A Bang

No, The Universe Cannot Be A Billion Years Younger Than We Think

There really is a cosmic conundrum about how fast the Universe is expanding. Changing its age won’t help.

One of the most surprising and interesting discoveries of the 21st century is the fact that different methods of measuring the expansion rate of the Universe yield different, inconsistent answers. If you measure the expansion rate of the Universe by looking at the earliest signals — early density fluctuations in the Universe that were imprinted from the early stages of the Big Bang — you find that the Universe expands at one particular rate: 67 km/s/Mpc, with an uncertainty of about 1%.

On the other hand, if you measure the expansion rate using the cosmic distance ladder — by looking at astronomical objects and mapping their redshifts and distances — you get a different answer: 73 km/s/Mpc, with an uncertainty of about 2%. This really is a fascinating cosmic conundrum, but despite claims by one team to the contrary, you cannot fix it by making the Universe a billion years younger. Here’s why.

The expanding Universe, full of galaxies and the complex structure we observe today, arose from a smaller, hotter, denser, more uniform state. It took thousands of scientists working for hundreds of years for us to arrive at this picture, and yet the lack of a consensus on what the expansion rate actually is tells us that either something is dreadfully wrong, we have an unidentified error somewhere, or there’s a new scientific revolution just on the horizon. (C. FAUCHER-GIGUÈRE, A. LIDZ, AND L. HERNQUIST, SCIENCE 319, 5859 (47))

At first glance, you might think that the expansion rate of the Universe has everything to do with how old the Universe is. After all, if we go back to the moment of the hot Big Bang, and we know the Universe was expanding extremely rapidly from this hot, dense, state, we know it must have cooled and slowed as it expanded. The amount of time that has passed since the Big Bang, along with the ingredients (like radiation, normal matter, dark matter and dark energy) it’s made of, determine how fast the Universe should be expanding today.

If it expands 9% faster than we previously suspected, then perhaps the Universe is 9% younger than we’d anticipated. This is the naive (and incorrect) reasoning applied to the problem, but the Universe isn’t as simple as that.

Three different types of measurements, distant stars and galaxies, the large scale structure of the Universe, and the fluctuations in the CMB, enable us to reconstruct the expansion history of our Universe. The fact that different methods of measurement point to different expansion histories may point the way forward to a new discovery in physics, or a greater understanding of what makes up our Universe. (ESA/HUBBLE AND NASA, SLOAN DIGITAL SKY SURVEY, ESA AND THE PLANCK COLLABORATION)

The reason you cannot simply do this is that there are three independent pieces of evidence that have to all fit together in order to explain the Universe.

  1. You must consider the early relic data, from features (known as baryon acoustic oscillations, which represent interactions between normal matter and radiation) that appear in the large-scale structure of the Universe and the fluctuations in the cosmic microwave background.
  2. You must consider the distance ladder data, which uses the apparent brightnesses and measured redshifts of objects to reconstruct both the expansion rate and the change in the expansion rate over time throughout our cosmic history.
  3. And, finally, you must consider the stars and star clusters we know of in our galaxy and beyond, which can have the ages of their stars independently determined through astronomical properties alone.
Constraints on dark energy from three independent sources: supernovae, the CMB (cosmic microwave background) and BAO (which is a wiggly feature seen in the correlations of large-scale structure). Note that even without supernovae, we’d need dark energy for certain, and also that there are uncertainties and degeneracies between the amount of dark matter and dark energy that we’d need to accurately describe our Universe. (SUPERNOVA COSMOLOGY PROJECT, AMANULLAH, ET AL., AP.J. (2010))

If we look at the first two pieces of evidence — the early relic data and the distance ladder data — this is where the huge discrepancy in the expansion rate comes from. You can determine the expansion rate from both, and this is where the 9% inconsistency comes from.

But this is not the end of the story; not even close. You can see, from the graph above, that the distance ladder data (which includes the supernova data, in blue) and the early relic data (which is based on both baryon acoustic oscillations and cosmic microwave background data, in the other two colors) not only intersect and overlap, but that there are uncertainties in both the dark matter density (x-axis) and dark energy density (y-axis). If you have a Universe with more dark energy, it’s going to appear older; if you have a Universe with more dark matter; it’s going to appear younger.

Four different cosmologies lead to the same fluctuations in the CMB, but measuring a single parameter independently (like H_0) can break that degeneracy. Cosmologists working on the distance ladder hope to develop a similar pipeline-like scheme to see how their cosmologies are dependent on the data that is included or excluded. (MELCHIORRI, A. & GRIFFITHS, L.M., 2001, NEWAR, 45, 321)

This is the big issue when it comes to the early relic data and the distance ladder data: the data that we have can fit multiple possible solutions. A slow expansion rate can be consistent with a Universe with the fluctuations we see in the cosmic microwave background, for example (shown above), if you tweak the normal matter, dark matter, and dark energy densities, along with the curvature of the Universe.

In fact, if you look at the cosmic microwave background data alone, you can see that a larger expansion rate is very much possible, but that you need a Universe with less dark matter and more dark energy to account for it. What’s particularly interesting, in this scenario, is that even if you demand a higher expansion rate, the act of increasing the dark energy and decreasing the dark matter keeps the age of the Universe practically unchanged at 13.8 billion years.

Before Planck, the best-fit to the data indicated a Hubble parameter of approximately 71 km/s/Mpc, but a value of approximately 69 or above would now be too great for both the dark matter density (x-axis) we’ve seen via other means and the scalar spectral index (right side of the y-axis) that we require for the large-scale structure of the Universe to make sense. A higher value of the Hubble constant of 73 km/s/Mpc is still allowed, but only if the scalar spectral index is high, the dark matter density is low, and the dark energy density is high. (P.A.R. ADE ET AL. AND THE PLANCK COLLABORATION (2015))

If we work out the math where the Universe has the following parameters:

  • an expansion rate of 67 km/s/Mpc,
  • a total (normal+dark) matter density of 32%,
  • and a dark energy density of 68%,

we get a Universe that’s been around for 13.81 billion years since the Big Bang. The scalar spectral index (ns), in this case, is approximately 0.962.

On the other hand, if we demand that the Universe have the following very different parameters:

  • an expansion rate of 73 km/s/Mpc,
  • a total (normal+dark) matter density of 24%,
  • and a dark energy density of 76%,

we get a Universe that’s been around for 13.72 billion years since the Big Bang. The scalar spectral index (ns), in this case, is approximately 0.995.

Correlations between certain aspects of the magnitude of temperature fluctuations (y-axis) as a function of decreasing angular scale (x-axis) show a Universe that is consistent with a scalar spectral index of 0.96 or 0.97, but not 0.99 or 1.00. (P.A.R. ADE ET AL. AND THE PLANCK COLLABORATION)

Sure, the data we have for the scalar spectral index disfavors this value, but that’s not the point. The point is this: making the Universe expand faster does not imply a younger Universe. Instead, it implies a Universe with a different ratio of dark matter and dark energy, but the age of the Universe remains largely unchanged.

This is very different from what one team has been asserting, and it’s extremely important for a reason we’ve already brought up: the Universe must be at least as old as the stars within it. Although there are certainly substantial error bars (i.e., uncertainties) on the ages of any individual star or star cluster, the full suite of evidence cannot be reconciled very easily with a Universe that’s younger than about 13.5 billion years.

Located around 4,140 light-years away in the galactic halo, SDSS J102915+172927 is an ancient star that contains just 1/20,000th the heavy elements the Sun possesses, and should be over 13 billion years old: one of the oldest in the Universe, and having possibly formed before even the Milky Way. The existence of stars like this informs us that the Universe cannot have properties that lead to an age younger than the stars within it. (ESO, DIGITIZED SKY SURVEY 2)

It takes at least 50-to-100 million years for the Universe to form the first stars of all, and those stars were made of hydrogen and helium alone: they no longer exist today. Instead, the oldest individual stars are found in the outskirts of halos of individual galaxies, and have extraordinarily tiny amounts of heavy elements. These stars are, at best, part of the second generation of stars to form, and their ages are inconsistent with a Universe that’s a billion years younger than the accepted, best-fit 13.8 billion year figure.

But we can go beyond individual stars and look at the ages of globular clusters: dense collections of stars that formed back in our Universe’s early stages. The stars inside, based on which ones have turned into red giants and which ones have yet to do so, give us a completely independent measurement of the Universe’s age.

The twinkling stars you see are evidence of variability, which is due to a unique period/brightness relationship. This is an image of a portion of the globular cluster Messier 3, and the properties of the stars inside it allow us to determine the overall cluster’s age. (JOEL D. HARTMAN)

The science of astronomy began with the studies of the objects in the night sky, and no object is more numerous or apparent to the naked eye than the stars. Through centuries of study, we’ve learned one of the most essential pieces of astronomical science: how stars live, burn through their fuel, and die.

In particular, we know that all stars, when they’re alive and burning through their main fuel (fusing hydrogen into helium), have a specific brightness and color, and remain at that specific brightness and color only for a certain amount of time: until their cores start to run out of fuel. At that point, the brighter, bluer and higher mass stars begin to “turn off” of the main sequence (the curved line on the color-magnitude diagram, below), evolving into giants and/or supergiants.

The life cycles of stars can be understood in the context of the color/magnitude diagram shown here. As the population of stars age, they ‘turn off’ the diagram, allowing us to date the age of the cluster in question. The oldest globular star clusters have an age of at least 13.2 billion years. (RICHARD POWELL UNDER C.C.-BY-S.A.-2.5 (L); R. J. HALL UNDER C.C.-BY-S.A.-1.0 (R))

By looking at where that turn-off-point is for a cluster of stars that all formed at the same time, we can figure out — if we know how stars work — how old those stars in the cluster are. When we look at the oldest globular clusters out there, the ones lowest in heavy elements and whose turn-offs come for the lowest-mass stars out there, many are older than 12 or even 13 billion years, with ages up to around 13.2 billion years.

There are none that are older than the currently accepted age of the Universe, which seems to provide an important consistency check. The objects we see in the Universe would have a tremendously hard time reconciling with an age of the Universe of 12.5 billion years, which is what you’d get if you lowered our best-fit figure (of 13.8 billion years) by 9%. A younger Universe is, at best, a cosmic long-shot.

Modern measurement tensions from the distance ladder (red) with early signal data from the CMB and BAO (blue) shown for contrast. It is plausible that the early signal method is correct and there’s a fundamental flaw with the distance ladder; it’s plausible that there’s a small-scale error biasing the early signal method and the distance ladder is correct, or that both groups are right and some form of new physics (shown at top) is the culprit. But right now, we cannot be sure.(ADAM RIESS (PRIVATE COMMUNICATION))

There may be some who contend we don’t know what the age of the Universe is, and that this conundrum over the expanding Universe could result in a Universe much younger than what we have today. But that would invalidate a large amount of robust data we already have and accept; a far more likely resolution is that the dark matter and dark energy densities are different than we previously suspected.

Travel the Universe with astrophysicist Ethan Siegel. Subscribers will get the newsletter every Saturday. All aboard!

Something interesting is surely going on with the Universe to provide us with such a fantastic discrepancy. Why does the Universe seem to care which technique we use to measure the expansion rate? Is dark energy or some other cosmic property changing over time? Is there a new field or force? Does gravity behave differently on cosmic scales than expected? More and better data will help us find out, but a significantly younger Universe is unlikely to be the answer.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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