I – II – III – IV – V – VI – VII – VIII – IX – X – XI – XII – XIII – XIV – XV – XVI – XVII – XVIII – XIX – XX – XXI – XXII – XXIII – XXIV – XXV – XXVI – XXVII – XXVIII – XXIX – XXX – XXXI – XXXII – XXXIII – XXXIV – XXXV – XXXVI – XXXVII – XXXVIII – XXXIX – XL – XLI – XLII – XLIII – XLIV – XLV – XLVI
Biblical literalism, of course, insists that the earth is only 6,000 years old, not 4.6 billion – so, naturally, it’s understandable that someone who thinks the world is that young might have an especially hard time believing that such dramatic evolutionary change could occur over such a short time frame. It’s one thing to imagine that humans could have evolved over the course of four billion years, but it’s another thing entirely to imagine such a process taking place in just 6,000 years. That being said, though, this is no reason to think that evolution must therefore be false; on the contrary, it’s all the more reason to think that the Bible and its estimation of the earth’s age must be mistaken.
And sure enough, when we look at all the available evidence, that’s what we see. As with evolution, every relevant scientific finding indicates that the earth is most definitely not as young as the Bible suggests – and as with evolution, the evidence comes from multiple independent lines of inquiry within multiple different fields (geology, physics, astronomy, etc.), all of which corroborate each other perfectly and point to the exact same 4.6 billion year timeline.
Potholer54 provides another useful clip here, with a quick summary of the most relevant evidence:
And Underlings gives a few more additions to boot (starting at the 9:33 mark):
The evidence against a 6,000-year timeline becomes even more overwhelming if we expand our inquiry to include not just the age of the earth (4.6 billion years), but the age of the rest of the universe as well (13.8 billion years). As NASA’s website explains:
Astronomers estimate the age of the universe in two ways: 1) by looking for the oldest stars; and 2) by measuring the rate of expansion of the universe and extrapolating back to the Big Bang; just as crime detectives can trace the origin of a bullet from the holes in a wall.
Astronomers can place a lower limit to the age of the universe by studying globular clusters. Globular clusters are a dense collection of roughly a million stars. Stellar densities near the center of the globular cluster are enormous. If we lived near the center of one, there would be several hundred thousand stars closer to us than Proxima Centauri, the star nearest to the Sun.
The life cycle of a star depends upon its mass. High mass stars are much brighter than low mass stars, thus they rapidly burn through their supply of hydrogen fuel. A star like the Sun has enough fuel in its core to burn at its current brightness for approximately 9 billion years. A star that is twice as massive as the Sun will burn through its fuel supply in only 800 million years. A 10 solar mass star, a star that is 10 times more massive than the Sun, burns nearly a thousand times brighter and has only a 20 million year fuel supply. Conversely, a star that is half as massive as the Sun burns slowly enough for its fuel to last more than 20 billion years.
All of the stars in a globular cluster formed at roughly the same time, thus they can serve as cosmic clocks. If a globular cluster is more than 20 million years old, then all of its hydrogen burning stars will be less massive than 10 solar masses. This implies that no individual hydrogen burning star will be more than 1000 times brighter than the Sun. If a globular cluster is more than 2 billion years old, then there will be no hydrogen-burning star more massive than 2 solar masses.
The oldest globular clusters contain only stars less massive than 0.7 solar masses. These low mass stars are much dimmer than the Sun. This observation suggests that the oldest globular clusters are between 11 and 18 billion years old.
Measurements by the WMAP satellite can [also] help determine the age of the universe. The detailed structure of the cosmic microwave background fluctuations depends on the current density of the universe, the composition of the universe and its expansion rate. As of 2013, WMAP determined these parameters with an accuracy of better than 1.5%. In turn, knowing the composition with this precision, we can estimate the age of the universe to about 0.4%: 13.77 ± 0.059 billion years!
How does WMAP data enable us to determine the age of the universe is 13.77 billion years, with an uncertainty of only 0.4%? The key to this is that by knowing the composition of matter and energy density in the universe, we can use Einstein’s General Relativity to compute how fast the universe has been expanding in the past. With that information, we can turn the clock back and determine when the universe had “zero” size, according to Einstein. The time between then and now is the age of the universe.
The expansion age measured by WMAP is larger than the oldest globular clusters, so the Big Bang theory has passed an important test using data independent of the type collected by WMAP. If the expansion age measured by WMAP had been smaller than the oldest globular clusters, then there would have been something fundamentally wrong about either the Big Bang theory or the theory of stellar evolution. Either way, astronomers would have needed to rethink many of their cherished ideas. But our current estimate of age fits well with what we know from other kinds of measurements.
And on top of these measurements, there are still more astronomical features that wouldn’t even be able to exist unless the universe was more than 6,000 years old. When certain stars die, for instance, they shed their outer layers (forming nebulae) and leave behind ultra-hot cores, which are called white dwarfs. Because these white dwarfs no longer have a source of nuclear fuel, they gradually radiate away the remainder of their energy and cool down. But because their surface area is so small, this process is extremely slow and gradual, taking billions of years to finish. By seeing how far along in this process a particular white dwarf is, scientists can determine its age – and as it turns out, there are a number of white dwarfs that are so far into their cooling cycles that they could only have arrived there after billions of years. (To use an analogy, it’s like having a piece of steel that has cooled down to room temperature. You might not have been there when it was forged, but you know that it must not have been forged within the last thirty seconds, because otherwise it would still be red-hot; therefore, you can correctly deduce that it must have existed for longer than thirty seconds.)
Similarly, there are certain types of high-energy astronomical bodies, like pulsars and quasars, that emit massive streams of ionized matter called astrophysical jets. These jets can stretch up to a million light years across – and because nothing can travel faster than the speed of light, that means that even if they were moving as fast as it’s possible for anything to move, it would have taken these jets at least a million years to have gotten from their sources (the quasars and pulsars) to where we see them today. Unless these celestial objects are somehow older than the universe itself, the universe can’t be less than a million years old.
And in that same vein, the fact that we even see distant stars and galaxies at all means that the universe must be billions of years old, because so many of these celestial objects are themselves billions of light years away from Earth – meaning that the light they emit takes billions of years to reach our eyes. When we look at a distant star or galaxy, what we’re seeing isn’t actually how that star or galaxy currently looks, but how it looked billions of years ago, when it originally emitted that light. The light had to travel across billions of light years to get from its source to Earth, and at the speed that light moves, that means that the universe has to have existed for billions of years in order to make such a trip possible in the first place.
The evidence doesn’t stop there; if you’re interested, RationalWiki has a whole long list you can check out. Underlings also has a great video compilation of everything mentioned here and more. The question remains, though – if the Big Bang really does explain the origin of the universe, then how did it actually happen? How is it possible that all the matter in the universe could have just spontaneously appeared out of nowhere? One of the most popular arguments against the Big Bang is that it’s impossible for something to simply “pop into existence” out of nothing – at least not without divine intervention.
But believe it or not, such a thing actually happens all the time; it just happens at the quantum scale (i.e. smaller than the smallest fundamental particles), so we never see it with our naked eyes. Explaining how this happens is complicated and involves a lot of math (and I’m not a physicist, so take everything I say on the subject with a grain of salt because I could be explaining it wrong) – but the basic idea is that an empty void isn’t actually an inherently stable state; as explained by Heisenberg’s uncertainty principle, there are always infinitesimal energy fluctuations at the quantum level. This means that certain special types of particles can (and do) spontaneously pop into existence out of nowhere, and scientists can measure their effects. But how is this possible? The key is that when one of these particles does appear, it’s always accompanied by a matching antiparticle – i.e. a particle with opposite physical charges – that cancels it out, so to speak, so the net amount of “stuff” within the system (including energy and charge and so on) remains at zero. (The particle and antiparticle quickly annihilate each other and disappear again, usually.) This might sound weird, but think of it this way: Imagine you have an equation like “zero equals positive ten plus negative ten.” How can there be such a thing as a “positive ten” in there, when the other side of the equation is zero? How can a positive quantity come out of nothing? But the answer is obvious, because that positive quantity is canceled out by a negative one of the same value, so the total is still zero. In the same way, a positive amount of matter or energy can come into existence, on a quantum scale of time and space, as long as it’s accompanied by an equal negative amount of matter or energy offsetting it. (I’m using matter and energy interchangeably here, because as Einstein’s famous E=mc2 equation shows, they’re actually just two different manifestations of the same thing, so under certain conditions one can be readily converted into the other.) When the Big Bang happened, then, it really would have been possible for quantum fluctuations to cause particles and antiparticles to appear out of nowhere. And from there, according to what’s called the cosmological inflation model, it would have been possible for an exponential expansion of space to occur in an infinitesimal fraction of a second. This instant of inflation (which is thought to have been triggered by the phase transition of the strong nuclear force separating from the other elementary forces in a near-instantaneous decay process), would have amplified those tiny quantum fluctuations to a classical scale, forming the seeds of all the matter and energy we see in the universe today. And time would have taken care of the rest; galaxies would have coalesced, planets would have formed, organisms would have evolved, and eventually intelligent beings would have emerged – all from the tiniest quantum fluctuations.
(Remarkably enough, we can actually still see the imprint of these quantum fluctuations today, in the cosmic microwave background radiation – the radiation left over from the Big Bang. When you turn on an old TV and see a bunch of static, what you’re seeing is actually (in part) the afterglow of the Big Bang itself!)
Again, this is an explanation of the origin of the universe that would only be possible if the total net mass/energy of the universe (positive energy plus negative energy) was zero. And up until recently, we didn’t know for sure whether the universe actually did have zero net energy or not; the evidence just wasn’t there yet. There were still some numbers that didn’t quite match up, some pieces of the puzzle that didn’t quite fit. But just twenty years ago, that changed; scientists discovered that the expansion of the universe was accelerating, and that the amount of energy needed to produce the detected rate of acceleration was – lo and behold – precisely the amount of energy that would bring the disparate numbers perfectly into line and put the net energy level of the universe at exactly zero. With that discovery, the pieces fell seamlessly into place; and we now have just the evidence we need to show that all the matter in the universe could, in fact, have spontaneously “appeared out of nowhere.” Lawrence Krauss gives a good summary of the whole venture here:
Of course, this isn’t quite the end of the story. After all, even if all the matter in the universe came from fluctuations in quantum fields stretching across spacetime, that still leaves the question of where spacetime itself came from. What happened before the Big Bang that caused time and space themselves to come into existence? Well, that’s actually a bit of a trick question – because the very concept of “before” is one that requires time to already exist. That’s why, according to cosmologists like Stephen Hawking, the question of “what came before the Big Bang?” doesn’t even make sense; it’s like asking what’s north of the North Pole. As Hawking puts it, time and space didn’t precede the Big Bang, they were themselves products of the Big Bang. There was no “moment before the Big Bang,” because the Big Bang itself was the first moment. Sean Carroll explains in slightly more depth:
The idea of the universe having a beginning – whether time is fundamental or emergent – suggests to some people that there must be something that brought it into being, and typically that something is identified with God. This intuition is codified in the cosmological argument for God’s existence, an idea that traces its lineage back at least as far as Plato and Aristotle. In recent years it has been championed by theologian William Lane Craig, who puts it in the form of a syllogism:
- Whatever begins to exist, has a cause.
- The Universe begins to exist.
- Therefore, the Universe had a cause.
[The] second premise of the argument may or may not be correct; we simply don’t know, as our current scientific understanding isn’t up to the task. The first premise is false. Talking about “causes” is not the right vocabulary to use when thinking about how the universe works at a deep level. We need to be asking ourselves not whether the universe had a cause but whether having a first moment in time is compatible with the laws of nature.
As we go through our lives, we don’t see random objects popping into existence. It might be forgivable to think that, at least with a high degree of credence, the universe itself shouldn’t simply pop into existence. But there are two very substantial mistakes lurking beneath that innocent-sounding idea.
The first mistake is that saying that the universe had a beginning is not the same as saying it popped into existence. The latter formulation, which is natural from an everyday point of view, leans heavily on a certain way of thinking about time. For something to pop into existence implies that at an earlier moment it was not there, and at a later moment it was. But when we’re talking about the universe, that “earlier” moment simply does not exist. There is not a moment in time where there is no universe, and another moment in time where there is; all moments in time are necessarily associated with an existing universe. The question is whether there can be a first such moment, an instant of time prior to which there were no other instants. That’s a question our intuitions just aren’t up to addressing.
Said another way: even if the universe has a first moment of time, it’s wrong to say that it “comes from nothing.” That formulation places into our mind the idea that there was a state of being, called “nothing,” which then transformed into the universe. That’s not right; there is no state of being called “nothing,” and before time began, there is no such thing as “transforming.” What there is, simply, is a moment of time before which there were no other moments.
The second mistake is to assert that things don’t simply pop into existence, rather than asking why that doesn’t happen in the world we experience [at least not at the macroscopic scale of everyday objects]. What makes me think that, despite my best wishes, a bowl of ice cream is not going to pop into existence right in front of me? The answer is that it would violate the laws of physics. Those include conservation laws, which say certain things remain constant over time, such as momentum and energy and electric charge. I can be fairly confident that a bowl of ice cream isn’t going to materialize in front of me because that would violate the conservation of energy.
Along those lines, it seems reasonable to believe that the universe can’t simply begin to exist, because it’s full of stuff, and that stuff has to come from somewhere. Translating that into physics-speak, the universe has energy, and energy is conserved – it’s neither created nor destroyed.
Which brings us to the important realization that makes it completely plausible that the universe could have had a beginning: as far as we can tell, every conserved quantity characterizing the universe (energy, momentum, charge) is exactly zero.
It’s not surprising that the electric charge of the universe is zero. Protons have a positive charge, electrons have an equal but opposite negative charge, and there seem to be equal numbers of them in the universe, adding up to a total charge of zero. But claiming that the energy of the universe is zero is something else entirely. There are clearly many things in the universe that have positive energy. So to have zero energy overall, there would have to be something with negative energy – what is that?
The answer is “gravity.” In general relativity, there is a formula for the energy of the whole universe at once. And it turns out that a uniform universe – one in which matter is spread evenly through space on very large scales – has precisely zero energy. The energy of “stuff” like matter and radiation is positive, but the energy associated with the gravitational field (the curvature of spacetime) is negative, and exactly enough to cancel the positive energy in the stuff.
If the universe had a nonzero amount of some conserved quantity like energy or charge, it couldn’t have an earliest moment in time – not without violating the laws of physics. The first moment of such a universe would be one in which energy or charge existed without any previous existence, which is against the rules. But as far as we know, our universe isn’t like that. There seems to be no obstacle in principle to a universe like ours simply beginning to exist.
Mind you, this isn’t to say that this explanation has been definitively proven, or that there’s no room for other possibilities. Scientists still take seriously the possibility that time and space might have existed before the Big Bang – because although we can safely say that the Big Bang was the beginning of our universe, that doesn’t necessarily mean it was the beginning of everything in existence. It’s possible, for instance, that our universe might actually be a smaller “bubble” amidst a larger multitude of universes. It’s possible that it was born from a black hole inside another universe. It’s even possible that the Big Bang really might have been initiated by a higher intelligence, like some alien programmer with a universe-simulating machine, or some other ultra-powerful being that we might consider godlike. That still wouldn’t solve the problem of where existence itself came from, mind you – you’d still have to somehow explain the origins of whichever realm the creator(s) resided in – but it’s at least a theoretical possibility.
All this is just to say, then, that the current scientific understanding of the Big Bang isn’t a totally finished one – not by a long shot. (If science had the perfect final answer to every question already, it would no longer have anything to do!) Science is a process, not a fixed set of answers – so although we know that the Big Bang happened, and we know when, there’s a lot that’s still being discovered about exactly how and why it happened (and whether inflation happened as we understand it, etc.). There are still plenty of exciting questions out there for scientists to discover the answers to, and the process is an ongoing one. But that’s the whole beauty of science; when it encounters interesting new questions, it doesn’t just assert whichever answer feels the most right and then refuse to ever change it – it goes out and tries to find out what the real answer is, and it keeps updating its knowledge base on a continual basis as new evidence is discovered.