Turns Out There Is No Actual Looking Up

Direction is something we humans are pretty accustomed to. Living in our friendly terrestrial environment, we are used to seeing things in term of up and down, left and right, forwards or backwards. And to us, our frame of reference is fixed and doesn’t change, unless we move or are in the process of moving. But when it comes to cosmology, things get a little more complicated.

For a long time now, cosmologists have held the belief that the universe is homogeneous and isotropic – i.e. fundamentally the same in all directions. In this sense, there is no such thing as “up” or “down” when it comes to space, only points of reference that are entirely relative. And thanks to a new study by researchers from the University College London, that view has been shown to be correct.

For the sake of their study, titled “How isotropic is the Universe?“, the research team used survey data of the Cosmic Microwave Background (CMB) – the thermal radiation left over from the Big Bang. This data was obtained by the ESA’s Planck spacecraft between 2009 and 2013.

The cosmic microwave background radiation, enhanced to show the anomalies. Credit: ESA and the Planck Collaboration
The cosmic microwave background radiation, enhanced to show the anomalies. Credit: ESA and the Planck Collaboration

The team then analyzed it using a supercomputer to determine if there were any polarization patterns that would indicate if space has a “preferred direction” of expansion. The purpose of this test was to see if one of the basic assumptions that underlies the most widely-accepted cosmological model is in fact correct.

The first of these assumptions is that the Universe was created by the Big Bang, which is based on the discovery that the Universe is in a state of expansion, and the discovery of the Cosmic Microwave Background. The second assumption is that space is homogenous and istropic, meaning that there are no major differences in the distribution of matter over large scales.

This belief, which is also known as the Cosmological Principle, is based partly on the Copernican Principle (which states that Earth has no special place in the Universe) and Einstein’s Theory of Relativity – which demonstrated that the measurement of inertia in any system is relative to the observer.

This theory has always had its limitations, as matter is clearly not evenly distributed at smaller scales (i.e. star systems, galaxies, galaxy clusters, etc.). However, cosmologists have argued around this by saying that fluctuation on the small scale are due to quantum fluctuations that occurred in the early Universe, and that the large-scale structure is one of homogeneity.

Timeline of the Big Bang and the expansion of the Universe. Credit: NASA
Timeline of the Big Bang and the expansion of the Universe. Credit: NASA

By looking for fluctuations in the oldest light in the Universe, scientists have been attempting to determine if this is in fact correct. In the past thirty years, these kinds of measurements have been performed by multiple missions, such as the Cosmic Background Explorer (COBE) mission, the Wilkinson Microwave Anisotropy Probe (WMAP), and the Planck spacecraft.

For the sake of their study, the UCL research team – led by Daniela Saadeh and Stephen Feeney – looked at things a little differently. Instead of searching for imbalances in the microwave background, they looked for signs that space could have a preferred direction of expansion, and how these might imprint themselves on the CMB.

As Daniela Saadeh – a PhD student at UCL and the lead author on the paper – told Universe Today via email:

“We analyzed the temperature and polarization of the cosmic microwave background (CMB), a relic radiation from the Big Bang, using data from the Planck mission. We compared the real CMB against our predictions for what it would look like in an anisotropic universe. After this search, we concluded that there is no evidence for these patterns and that the assumption that the Universe is isotropic on large scales is a good one.”

Basically, their results showed that there is only a 1 in 121 000 chance that the Universe is anisotropic. In other words, the evidence indicates that the Universe has been expanding in all directions uniformly, thus removing any doubts about their being any actual sense of direction on the large-scale.

Now and Then. This single all-sky image simultaneously captured two snapshots that straddle virtually the entire 13.7 billion year history of the universe. One of them is ‘now’ – our galaxy and its structures seen as they are over the most recent tens of thousands of years (the thin strip extending across the image is the edge-on plane of our galaxy – the Milky Way). The other is ‘then’ – the red afterglow of the Big Bang seen as it was just 380,000 years after the Big Bang (top and bottom of image). The time between these two snapshots therefore covers about 99.997% of the 13.7 billion year age of the universe. The image was obtained by the Planck spacecraft. Credit: ESA
A “now and then” all-sky image captured by the Planck spacecraft, simultaneously showing our galaxy and its structures seen as in recent history; and ‘then’ – the red afterglow of the Big Bang seen as it was just 380,000 years later. Credit: ESA

And in a way, this is a bit disappointing, since a Universe that is not homogenous and the same in all directions would lead to a set of solutions to Einstein’s field equations. By themselves, these equations do not impose any symmetries on space time, but the Standard Model (of which they are part) does accept homogeneity as a sort of given.

These solutions are known as the Bianchi models, which were proposed by Italian mathematician Luigi Bianchi in the late 19th century. These algebraic theories, which can be applied to three-dimensional spacetime, are obtained by being less restrictive, and thus allow for a Universe that is anisotropic.

On the other hand, the study performed by Saadeh, Feeney, and their colleagues has shown that one of the main assumptions that our current cosmological models rest on is indeed correct. In so doing, they have also provided a much-needed sense of closer to a long-term debate.

“In the last ten years there has been considerable discussion around whether there were signs of large-scale anisotropy lurking in the CMB,” said Saadeh. “If the Universe were anisotropic, we would need to revise many of our calculations about its history and content. Planck high-quality data came with a golden opportunity to perform this health check on the standard model of cosmology and the good news is that it is safe.”

So the next time you find yourself looking up at the night sky, remember… that’s a luxury you have only while you’re standing on Earth. Out there, its a whole ‘nother ballgame! So enjoy this thing we call “direction” when and where  you can.

And be sure to check out this animation produced by the UCL team, which illustrates the Planck mission’s CMB data:

Further Reading: arXiv, Science

Astronomy Without A Telescope – The Edge Of Significance

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Some recent work on Type 1a supernovae velocities suggests that the universe may not be as isotropic as our current standard model (LambdaCDM) requires it to be.

The standard model requires the universe to be isotropic and homogeneous – meaning it can be assumed to have the same underlying structure and principles operating throughout and it looks measurably the same in every direction. Any significant variation from this assumption means the standard model can’t adequately describe the current universe or its evolution. So any challenge to the assumption of isotropy and homogeneity, also known as the cosmological principle, is big news.

Of course since you are hearing about such a paradigm-shifting finding within this humble column, rather than as a lead article in Nature, you can safely assume that the science is not quite bedded down yet. The Union2 data set of 557 Type 1a supernovae, released in 2010, is allegedly the source of this latest challenge to the cosmological principle – even though the data set was released with the unequivocal statement that the flat concordance LambdaCDM model remains an excellent fit to the Union2 data.

Anyhow, in 2010 Antoniou and Perivolaropoulos ran a hemisphere comparison – essentially comparing supernova velocities in the northern hemisphere of the sky with the southern hemisphere. These hemispheres were defined using galactic coordinates, where the orbital plane of the Milky Way is set as the equator and the Sun, which is more or less on the galactic orbital plane, is the zero point.

The galactic coordinate system. Credit: thinkastronomy.com

Antoniou and Perivolaropoulos’ analysis determined a preferred axis of anisotropy – with more supernovae showing higher than average velocities towards a point in the northern hemisphere (within the same ranges of redshift). This suggests that a part of the northern sky represents a part of the universe that is expanding outwards with a greater acceleration than elsewhere. If correct, this means the universe is neither isotropic nor homogeneous.

However, they note that their statistical analysis does not necessarily correspond with statistically significant anisotropy and then seek to strengthen their finding by appealing to other anomalies in cosmic microwave background data which also show anisotropic tendencies. So this seems to be a case of looking at number of unrelated findings with common trends – that in isolation are not statistically significant – and then arguing that if you put all these together they somehow achieve a consolidated significance that they did not possess in isolation.

More recently, Cai and Tuo ran much the same hemispherical analysis and, not surprisingly, got much the same result. They then tested whether these data favoured one dark energy model over another – which they didn’t. Nonetheless, on the strength of this, Cai and Tuo gained a write up in the Physics Arxiv blog under the heading More Evidence for a Preferred Direction in Spacetime – which seems a bit of a stretch since it’s really just the same evidence that has been separately analysed for another purpose.

It’s reasonable to doubt that anything has been definitively resolved at this point. The weight of current evidence still favours an isotropic and homogeneous universe. While there’s no harm in mucking about at the edge of statistical significance with whatever limited data are available – such fringe findings may be quickly washed away when new data comes in – e.g. more Type 1a supernovae velocity measures from a new sky survey – or a higher resolution view of the cosmic microwave background from the Planck spacecraft. Stay tuned.

Further reading:
– Antoniou and Perivolaropoulos. Searching for a Cosmological Preferred Axis: Union2 Data Analysis and Comparison with Other Probes.
– Cai and Tuo. Direction Dependence of the Deceleration Parameter.

Astronomy Without A Telescope – Assumptions

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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?

How Many Atoms Are There in the Universe?

It’s no secret that the universe is an extremely vast place. That which we can observe (aka. “the known Universe”) is estimated to span roughly  93 billion light years. That’s a pretty impressive number, especially when you consider its only what we’ve observed so far. And given the sheer volume of that space, one would expect that the amount of matter contained within would be similarly impressive.

But interestingly enough, it is when you look at that matter on the smallest of scales that the numbers become the most mind-boggling. For example, it is believed that between 120 to 300 sextillion (that’s 1.2 x 10²³ to 3.0 x 10²³) stars exist within our observable universe. But looking closer, at the atomic scale, the numbers get even more inconceivable.

At this level, it is estimated that the there are between 1078 to 1082 atoms in the known, observable universe. In layman’s terms, that works out to between ten quadrillion vigintillion and one-hundred thousand quadrillion vigintillion atoms.

And yet, those numbers don’t accurately reflect how much matter the universe may truly house. As stated already, this estimate accounts only for the observable universe which reaches 46 billion light years in any direction, and is based on where the expansion of space has taken the most distant objects observed.

The history of theA billion years after the big bang, hydrogen atoms were mysteriously torn apart into a soup of ions.universe starting the with the Big Bang. Image credit: grandunificationtheory.com
The history of the universe starting the with the Big Bang. Image credit: grandunificationtheory.com

While a German supercomputer recently ran a simulation and estimated that around 500 billion galaxies exist within range of observation, a more conservative estimate places the number at around 300 billion. Since the number of stars in a galaxy can run up to 400 billion, then the total number of stars may very well be around 1.2×1023  – or just over 100 sextillion.

On average, each star can weigh about 1035 grams. Thus, the total mass would be about 1058 grams (that’s 1.0 x 1052 metric tons). Since each gram of matter is known to have about 1024 protons, or about the same number of hydrogen atoms (since one hydrogen atom has only one proton), then the total number of hydrogen atoms would be roughly 1086 – aka. one-hundred thousand quadrillion vigintillion.

Within this observable universe, this matter is spread homogeneously throughout space, at least when averaged over distances longer than 300 million light-years. On smaller scales, however, matter is observed to form into the clumps of hierarchically-organized luminous matter that we are all familiar with.

In short, most atoms are condensed into stars, most stars are condensed into galaxies, most galaxies into clusters, most clusters into superclusters and, finally, into the largest-scale structures like the Great Wall of galaxies (aka. the Sloan Great Wall). On a smaller scale, these clumps are permeated by clouds of dust particles, gas clouds, asteroids, and other small clumps of stellar matter.

Representation of the timeline of the universe over 13.7 billion years, and the expansion in the universe that followed. Credit: NASA/WMAP Science Team.
Representation of the timeline of the universe over 13.7 billion years, and the expansion in the universe that followed. Credit: NASA/WMAP Science Team.

The observable matter of the Universe is also spread isotropically; meaning that no direction of observation seems different from any other and each region of the sky has roughly the same content. The Universe is also bathed in a wave of highly isotropic microwave radiation that corresponds to a thermal equilibrium of roughly 2.725 kelvin (just above Absolute Zero).

The hypothesis that the large-scale universe is homogeneous and isotropic is known as the cosmological principle. This states that physical laws act uniformly throughout the universe and should, therefore, produce no observable irregularities in the large scale structure. This theory has been backed up by astronomical observations which have helped to chart the evolution of the structure of the universe since it was initially laid down by the Big Bang.

The current consensus amongst scientists is that the vast majority of matter was created in this event, and that the expansion of the Universe since has not added new matter to the equation. Rather, it is believed that what has been taking place for the past 13.7 billion years has simply been an expansion or dispersion of the masses that were initially created. That is, no amount of matter that wasn’t there in the beginning has been added during this expansion.

However, Einstein’s  equivalence of mass and energy presents a slight complication to this theory. This is a consequence arising out of Special Relativity, in which the addition of energy to an object increases its mass incrementally. Between all the fusions and fissions, atoms are regularly converted from particles to energies and back again.

Atom density is greater at left (the beginning of the experiment) than 80 milliseconds after the simulated Big Bang. Credit: Chen-Lung Hung
Atom density is greater at left (the beginning of the experiment) than 80 milliseconds after the simulated Big Bang. Credit: Chen-Lung Hung

Nevertheless, observed on a large-scale, the overall matter density of the universe remains the same over time. The present density of the observable universe is estimated to be very low – roughly 9.9 × 10-30 grams per cubic centimeter. This mass-energy appears to consist of 68.3% dark energy, 26.8% dark matter and just 4.9% ordinary (luminous) matter. Thus the density of atoms is on the order of a single hydrogen atom for every four cubic meters of volume.

The properties of dark energy and dark matter are largely unknown, and could be uniformly distributed or organized in clumps like normal matter. However, it is believed that dark matter gravitates as ordinary matter does, and thus works to slow the expansion of the Universe. By contrast, dark energy accelerates its expansion.

Once again, this number is just a rough estimate. When used to estimate the total mass of the Universe, it often falls short of what other estimates predict. And in the end, what we see is just a smaller fraction of the whole.

We’ve got a many articles that are related to the amount of matter in the Universe here in Universe Today, like How Many Galaxies in the Universe, and How Many Stars are in the Milky Way?

NASA also has the following articles on the universe, like How many galaxies are there? and this article on the Stars in Our Galaxy.

We also have podcast episodes from Astronomy Cast on the subject of Galaxies and Variable Stars.