In response to the call for uncomfortable questions, Jason Failes asks:
What's the best evidence for the Big Bang theory?
The more I read about it (25 years ago to present), the more contrived, ad hoc, and retro-dictive it seems.
At this point, what would falsify the Big Bang theory?
What would falsify the Big Bang? Jesus Christ his own self turning up at the American Astronomical Society meeting, turning water to wine, and giving a talk titled "What Big Bang? How I Hoaxed You All."
OK, that's pretty flippant, but my understanding of the matter is that the evidence supporting the current cosmological model is simply overwhelming. The measured redshifts of distant galaxies correspond very nicely to Hubble's Law, the cosmic microwave background radiation is at a temperature that fits perfectly (and, it should be noted, Ralph Alpher predicted the existence and temperature of the microwave background to within a factor of 2 in the late 1940's, nearly twenty years before it was discovered, so it's not all "retro-dictive"), observed abundances of light elements are right where you would expect, etc.
The only observation that isn't entirely consistent with a fairly straightforward variant of the original Big Bang idea is "dark energy," which manifests as an apparent acceleration of the expansion of the universe. That's a significant gap, to be sure, but it's still a recent discovery as such things go, and I'm not aware of any astronomers who believe it represents an insuperable obstacle for Big Bang cosmology.
The tendentious phrasing of the question suggests a belief in some other alternative, but I'm not aware of any alternative theory that is taken seriously by anybody in the field. The evidence supporting the current Big Bang model is compelling enough that falsifying it would require something not far short of an authenticated miracle.
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Yeah, finding distant, or too close, galaxies that semm inconsistent with Hubble's law would do it, probly. It's a bit like the cosmology version of the pre-cambrian dog fossil. Although for distant galaxies, it's more difficult to cross reference using cepheids and supernovae, while at close range, the local group motion is more difficult to factor.
What you really end up with is; is this model really consistent with our observations? The cosmic distance ladder is always open to improvement.
I'm more concerned about the dark matter and dark energy issues. We have just read on SB about how the estimate of our galaxy's mass has just been majorly adjusted. A bit of digging finds this description of the research project recently publicised.
I think this question depends on what exactly you mean by the Big Bang theory. It isn't really a specific model and so will not be falsified by failing to quantitatively describe what we see precisely enough. In cosmology talks that I've gone to, no one refers to the Big Bang theory but rather the Standard Model of Cosmology, which has a certain period of inflation and various ratios of visible matter, dark matter and dark energy. This is a specific model which could be falsified if it were realized that dark energy didn't exist, or whatever. But the basic idea that the visible universe was in a dense, hot state sometime in the past is unlikely to go away.
Depends on what you mean by "Big Bang Theory" doesn't it? I mean, if you're talking about the observation that all of the matter in the universe is busily flying apart in a sort of on-going explotion, that's more of a Big Bang Fact. We can see it. If you're talking about the inference that it all must have been much closer together in the past, well that's pretty ironclad too.
If you're talking about specific physical and mathematical statements, you have to be more specific about which ones. As Rob Knop says the theory has evolved so much over the years that it almost seems like a mistake to keep using the same name for all of them. They're pretty much all falsifiable, though. And some of the more recent big bang theories are not all that well supported yet.
If you're talking about the idea that the universe sprang into existence from nothingness (or as a bubble in some larger multiverse, which itself ultimately sprang from nothingness) Well, that's probably not falsifiable. It comes into the category of speculation. We cannot know what happened before time began. We can't see past a singularity. Unless we create a new universe in the lab, we can't ever test our ideas of how universes come to be.
I went to a talk by Alan Guth, and he tried to be pretty clear about the fact that we have no theories that we can apply before the first few milliseconds of the universe. But the theories that describe what happened from those first milliseconds onward are pretty solid.
I think there's a real chance inflation (and really most other speculations as to the universe above about 1e17 K) will turn out to be wrong. But that's really the story just a tiny fraction of a second after the bang; everything past that is very solid.
What if the high-order multipole moments of the MBR painted a perfect image of Mickey Mouse?
I think the question is really screwed up. Observations have satisfied all the predictions of the Big Bang model that we have. (Other predictions are from refinements to the Big Bang model such as inflation.) It's true that good theories should be falsifiable, but it's not a requirement that they should produce an _endless_ stream of predictions with which they could be falsified.
The most important piece of evidence for the Big Bang is the correct prediction of the relative abundances of (the stable isotopes of) the light elements.
The CMBR is impressive, but nucleosynthesis took place at an earlier time, when the universe was 10^5 times hotter.
1. If all galaxies are moving away from each other just where was the spot in the universe that the Big Bang occurred?
2. If light from the very distant galaxies are taking billions of light years to reach us (I think the record is close to 14bn light years), that galaxy must have taken time to form and make light. Yet we are told that the Big Bang occurred between 10 and 15bn years ago.
1. If all galaxies are moving away from each other just where was the spot in the universe that the Big Bang occurred?
I am just a poor engineer monkey, but I believe the correct answer is, you're sitting on it. IE, the universe itself is getting bigger, not that the things in the universe are flying away from some point in space.
2. If light from the very distant galaxies are taking billions of light years to reach us (I think the record is close to 14bn light years), that galaxy must have taken time to form and make light. Yet we are told that the Big Bang occurred between 10 and 15bn years ago.
The scuttle butt I hear is that the age of the universe is pegged to a much narrower range, something like 14.7 billion years. So a few hundred million years is enough. I couldn't find this with a simple web search, but if my math is correct those farthest galaxies aren't moving away just quickly, the part of the universe they are in is moving away from us at very close to the speed of light.
Hello all.
First off, thanks to Professor Orzel for so quickly addressing this question that has vexed me for so very long.
Some details:
-The expansion of the universe is often cited as evidence for the BB theory, positing that since the universe is expanding, something must have begun that expansion, and that that something must be the hot, dense concentration of all matter and energy at the beginning of time.
However, the expansion of the universe is accelerating. Leaving aside for the moment a lengthy discussion of what Dark Energy might be, if large-scale structures are being pushed apart by some force today, why posit anything other than that force acting in the past? In other words, why have both the Big Bang and Dark Energy to explain the universe's expansion when Dark Energy alone may explain it?
Moreover, if the expansion is accelerating now, does this not mean that the expansion was slower in the past? Quite the opposite of the pre-1998 version of the BB theory, which had the universe expanding at a phenomenal rate at first (given a universe at least 93 billion light years across, expanding faster than the speed of light, which raises its own sizeable problems), then gradually slowing down by gravity over the passage of time, does not the current acceleration of the expansion indicate that the universe started out (if it "started out" at all) with a rate of expansion that was asymptotically close to zero, which then increased with time? Nothing could contradict the BB theory more thoroughly than the completely unexpected, yet clearly observed, acceleration of the universe's expansion.
At present, though, it is common to see both the BB theory's inflation and the completely contradictory observed acceleration crammed into one graph, the universe expanding rapidly from a point, then expansion slowed by gravity, then expansion accelerated by Dark Energy (like the universe is being run by a kid learning to drive standard for the first time).
-The Big Bang theory was not the only theory to predict the CMB, nor were its initial predictions the most accurate. As early as 1926, Sir Arthur Eddington estimated the minimum temperature of cold objects in space as 3K, based on cold objects re-radiating heat from infinite distant stars (he assumed a static universe or one going through periods of expansion and contraction). Meanwhile, various predictions of the CMB temperature based on the hot BB theory ranged from 3k all the way up to 50k. Much like the modern diversity of string theories, such diverse values were calculated that one of them was bound to be correct.
Now, of course, the static model failed, but four important points:
First, a static universe is not needed for Eddington's estimation to work, only great age and size. The universe is certainly very large, and given that the universe's acceleration is currently accelerating, and that Hubble's constant may have been smaller in the past, our time estimates of the universe's age may be a considerable underestimate.
Second, the CMB was not a unique prediction. This makes the common claim that the CMB is evidence for the BB unimpressive.
Third, the prediction does not flow naturally from the theory. If anything, the early higher predictions were more in line with a dense, hot, concentration of all matter and energy in the universe. All a 3k CMB tells us is that the universe is very cold and nearly homogeneous. How this has been linked to the BB has always been a mystery to me.
Forth, comparing the BB to the static state theory is a false dichotomy. The lack of other options likely has more to do with our own lack of knowledge and failures of imagination than it does the universe being limited to the ideas we've had to date.
-Old stars and large-scale structures are observed in the early universe.
-Large-scale structures are observed that should have taken longer than the estimated age of the universe to form.
-Limited observations, massive observational error bars, over-reliance on mathematics over empirical data, and the history of the failures of human cosmology all point to us being the traditionally overconfident and undersighted Mr.Magoos of the universe. It is unlikely that we have found the answer in the back of the book that easily, nor is it likely that the answer is really so comfortingly close to so many people's preferred creation myths.
As for my own ideas on the topic, I have no specific hypothesis; I just think that we've wasted the last decade since the accelerating expansion observations essentially killed the Big Bang theory. Instead of contorting the BB theory far out of its natural shape to fit the new observations, new ideas should have been explored.
As a general search strategy, I think we should be looking for a process that is simple and iterative, such as the effects of virtual particles in empty space over massive time frames. Any asymmetries at all could have massive implications for the formation of matter in the universe. That is just an example, but I can't help but think that there's a more beautiful and sensible explanation for all of this.
At this point the BB is far too much like the "and then a miracle happens" of the famous cartoon for both my liking and my intellect.
I apologize for the length of this post.
From Jason: At this point the BB is far too much like the "and then a miracle happens" of the famous cartoon for both my liking and my intellect.
I think you miss the point, and the basis of the theory. The Big Bang doesn't "explain" the observed expansion. It follows from tracing the expansion back in time in the context of general relativity. If anything can be said to explain the expansion it is the application of general relativity on a cosmological scale, ie, to the universe as a whole. Given certain assumptions about the distribution of matter and energy in the universe, which stand up quite well observationally, there are limited options for describing the overall dynamics of the universe. A lack of knowledge or imagination is not the issue.
The accelerating expansion is perfectly consistent with general relativity and the Big Bang, assuming one allows a non-zero and positive cosmological constant, a term in Einstein's field equation which is permitted by the assumptions underlying the derivation of the basic dynamics of the theory. Einstein actually initially postulated a non-zero value for the constant, hoping to produce a static cosmological solution, but concluded it was unnecessary, given the evidence of expansion accumulated by the 1930s. (The static cosmological solution was also inadequate, because it was unstable; slight perturbations would trigger expansion or contraction.)
The problem with the accelerating expansion is simply that it is so hard to understand a small but still non-zero positive value of the cosmological constant. In fact, this is one of the central unresolved problems in fundamental physics and cosmology. The so-called Big Bang Theory is no more "contrived" than general relativity itself.