Next Generation Science Standards

Performance Expectation

Construct an explanation of the Big Bang theory based on astronomical evidence of light spectra, motion of distant galaxies, and composition of matter in the universe.

• Clarification Statement: Emphasis is on the astronomical evidence of the red shift of light from galaxies as an indication that the universe is currently expanding, the cosmic microwave background as the remnant radiation from the Big Bang, and the observed composition of ordinary matter of the universe, primarily found in stars and interstellar gases (from the spectra of electromagnetic radiation from stars), which matches that predicted by the Big Bang theory (3/4 hydrogen and 1/4 helium).

Are your students prepared to make the case that our universe is expanding in the aftermath of a Big Bang that happened 13.8 billion years ago? If not, they're about to, because that's what this performance expectation asks of them.

Some students might think the Big Bang theory is just a TV show, while others might think it's a theory in the same sense that "the pyramids were made by Martians" is a theory, but that's where you come in. Armed with knowledge like the Doppler effect and cosmic background microwave radiation, students will have all the evidence they need to explain that the Big Bang theory isn't just some crackpot idea theoretical physicists came up with to keep themselves occupied.

These activity ideas will help you teach this standard with a bang:

• Show your students how to retrace Hubble's discovery of the Big Bang, or parts of that discovery, by crunching data from the Sloan Digital Sky Survey using the SkyServer website. The New Mexico-based telescope surveys redshift as part of an effort to map a ginormous swath of the universe. Anyone is invited to use the survey's findings, called the SkyServer database to prove the Hubble Law to themselves by creating a simple Hubble diagram.
• If they're ambitious, your students can also use SkyServer data to measure redshifts of distant galaxies on their own. They can also construct their own, more sophisticated Hubble diagrams using these findings to graph the log of the galaxy's velocity versus its magnitude.
• Make raisin bread with your students. Seriously. The rising dough is such a perfect analogy for the expanding universe that it's become a cliché. First off, it will illustrate the tricky point that the galaxies (think raisins) are moving farther apart with the surrounding space and not through space, because space (think dough) is what's expanding. The galaxies (raisins) don't actually ever leave behind the particular patch of dough they're mired in—they just move with that dough as it expands. Have your students record their observations, and the whole "moving-with-and-not-through" thang they witness should help them overcome a major stumbling block in comprehending this stuff: the mistaken notion that galaxies are moving away from a common center.
• If whipping up a batch of dough won't work in your classroom, get hold of some regular balloons. Before your students start blowing, though, have them draw plenty of dots on the latex. (Make sure no one's allergic!) Then have students measure the span between two dots close together and between two far apart. When they measure the spans between the same pairs on the inflated balloon, they'll see how the ones that were already far apart spread away from each other by a bigger factor than did the ones closer together. Have them record their findings to yield a glimpse of the Hubble Law in action.
• Have your students make science research posters that defend the Big Bang. They should include graphics and illustrations, written explanations, descriptions of the students' own activities, and analysis of any data (from, say, any of the activities above). Why not reward them on the due date with fresh-baked raisin bread for a nice Miss Frizzle-ish touch? Or, how about celebrating with some marked-up balloons?

Disciplinary Core Ideas

ESS1.A – The Universe and Its Stars: The study of stars' light spectra and brightness is used to identify compositional elements of stars, their movements, and their distances from Earth.

To understand how scientists determined that the universe is expanding, students first need to know that each element gives off a telltale signature light spectrum. This is true whether we're talking about a simple flame test in the lab, or whether we're talking about stars. Armed with this knowledge, all scientists need to do is analyze the spectrum of light emitted from a star to determine what it's made of (or more accurately, the percent composition of various elements in a star).

Taking that a step further, light spectra also allow scientists to figure out the luminosity of a star, and therefore its distance from Earth, by calculating the absolute magnitude. You don't need to get into Hertzsprung-Russell diagrams or Stefan's Law for this performance expectation. The important thing is that students understand the spectrum of a star is like its fingerprint, telling us what it's made of and how far away.

It can also tell us if a star is moving, which ties into the next disciplinary core idea for this performance expectation, so let's keep moving.

ESS1.A – The Universe and Its Stars: The Big Bang theory is supported by observations of distant galaxies receding from our own, of the measured composition of stars and non-stellar gases, and of the maps of spectra of the primordial radiation (cosmic microwave background) that still fills the universe.

This disciplinary core idea covers a lot, so let's go through it piece by piece, picking up where we left off with spectra.

Since we know what elements are inside any given class of stars, we have a baseline standard for what the spectra from stars should look like. In reality, though, we observe that the spectra of (most) stars are a little off. Specifically, the spectra are shifted in the direction of lower frequencies, or towards red in the visible spectrum (hence the term redshift). This shifting in the spectra of stars (and entire galaxies) is the primary evidence that the universe is expanding.

To get students to understand this connection, you'll want to cover the good 'ol Doppler Effect. You can remind them of the phenomenon of a siren passing by, and how the pitch gets lower as soon as the fire engine whizzes past. The same thing goes for light waves, except instead of changes in pitch, we get changes in color.

PBS has a helpful interactive on the Doppler effect, if you need an assist, and it even ties into light and stars. Once students get a grasp of this very observable phenomenon, the Big Bang will suddenly make a whole lot more sense.

Your students should also know that the farther away a galaxy is, the more stretched, and therefore redder, its light waves become. They'll also need to know that the farther a galaxy is from us the faster its patch of sky is moving away from our own, aka Hubble's Law.

Next up, students should learn about the residual microwave radiation left over from when the universe was a gassy infant, 13.8 million years ago. Sometimes a stroll helps with digestion, so walk your students through simplified version of the theorized events of the Big Bang. From that theoretical sequence, scientists predicted that there should be some sort of glow leftover from that initial bang. Turns out that glow does indeed exist, in the form of microwaves. Boom, that's evidence.

Last item to cover with students here is the composition of elements in the universe. The Big Bang theory also predicts the order in which elements were first created, starting with hydrogen and helium. By analyzing the spectra of stars and galaxies extremely far away, scientists are essentially able to look into the past, and have indeed found evidence that corresponds to their hypotheses about which elements were created first.

ESS1.A – The Universe and Its Stars: Other than the hydrogen and helium formed at the time of the Big Bang, nuclear fusion within stars produces all atomic nuclei lighter than and including iron, and the process releases electromagnetic energy. Heavier elements are produced when certain massive stars achieve a supernova stage and explode.

Compared to the first two disciplinary core ideas, this one is pretty straightforward. Students first need to know that hydrogen and helium, being the smallest and simplest atoms, were formed first during the Big Bang. All the other elements on the periodic table were (and continue to be) created by stars, or by the destruction of stars.

Specifically, elements from lithium to iron on the periodic table are made during nuclear fusion inside stars. From cobalt and up, the rest of the elements are created when giant stars get to the supernova stage and go kablooey. At least most of them. Some of the really heavy elements are made by the decomposition of even heavier elements, but they still only exist because stars exploded.

PS4.B – Electromagnetic Radiation: Atoms of each element emit and absorb characteristic frequencies of light. These characteristics allow identification of the presence of an element, even in microscopic quantities. (Secondary to HS-ESS1-2)

This core concept from the physical sciences should provide students with a bit more detail about spectra (and maybe give them nightmares about chemistry class). Basically, students should know that there are two types of spectra: absorption spectra and emission spectra. These two spectra refer to the colors (wavelengths) either absorbed or emitted by atoms corresponding to the energy differences between their energy levels.

Since each element has a different atomic number and mass, each element has unique absorption and emission spectra. You don't need to go into too much detail with atomic theory here, but students should definitely understand the basic gist: 1) When an atom absorbs energy, the electrons jump up an energy level and we see dark lines in the spectrum; and 2) when an atom gives off energy, the electrons drop an energy level and give off photons that appear a certain color.

These two spectra are like a photograph and its negative, and each element has its unique signature. This is what makes it possible to identify the composition of stars and galaxies. Who needs magic when you've got science, right?

Science and Engineering Practices

Constructing Explanations and Designing Solutions: Construct an explanation based on valid and reliable evidence obtained from a variety of sources (including students' own investigations, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.

Once your students have a firm grasp of the disciplinary core ideas, they'll have all the tools they need to articulate what the Big Bang is all about on their own. The goal here is for students to be able to explain in their own words why the Big Bang theory isn't some crackpot conspiracy theory. To do that, of course, they're going to have to rely heavily on evidence.

Their explanations should mention the evidence of redshifts, background microwave radiation, and the composition of stars. The actual format of their explanations is totally up to you, or better yet, totally up to them. You can go the route of written explanations, or something more engaging like posters, presentations, videos, you name it. Let them choose on their own, and you'll get a nice variety of projects to keep your eyes from glazing over while trying to read 30+ expository essays.

Science Models, Laws, Mechanisms, and Theories Explain Natural Phenomena: A scientific theory is a substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment and the science community validates each theory before it is accepted. If new evidence is discovered that the theory does not accommodate, the theory is generally modified in light of this new evidence.

Students should know that the Big Bang theory is…wait for it…a theory. Yep, it's right there in the name, but students might need some help differentiating a theory from a law or a hypothesis.

Theories are mainstays of science, and they always function as explanations for phenomena. In the case of the Big Bang theory, it's a theory that explains why the universe is expanding and where it came from in the first place.

The second important aspect of a scientific theory is that it not only explains some mystery, but is supported by gobs of evidence. In the case of the Big Bang theory, we have redshifts, background microwave radiation, and the spectra of stars, which tells us their composition. They all point us in the same direction—13.8 billion years thataway.

Crosscutting Concepts

Energy and Matter: Energy cannot be created or destroyed–only moved between one place and another place, between objects and/or fields, or between systems.

Students should understand that energy and matter are conserved, and that the Big Bang theory is an explanation of where are all the energy and matter in the universe came from in the first place. You don't need to get into the minutia of thermodynamics here, but students should get that all the energy and matter in existence started off in a condensed, mega-hot ball. Then the bang happened, and as the universe expanded and broke into pieces, it cooled down. This didn't happen because energy disappeared. Instead, it simply got dispersed.

Students should also know that at this stage in the lifespan of our universe, energy and matter is still continually moved around. The primary location for this is in stars, where nuclear reactions turn hydrogen and helium into light elements, and the explosion of some supernovae causes their lighter elements to turn into the heavier elements on the periodic table.

Interdependence of Science, Engineering, and Technology: Science and engineering complement each other in the cycle known as research and development (R&D). Many R&D projects may involve scientists, engineers, and others with wide ranges of expertise.

The general idea here as that your students should know that the relationship between science and technology is one of mutual benefit. Science gives engineers the knowledge they need to make better tools and technology, and that technology in turn makes it easier for scientists to do their work.

To give your students some perspective, explain to them that Edwin Hubble had to take one spectrograph at a time on photographic plates and record and analyze his data by hand. Not fun, but Hubble's discoveries made scientists realize they needed better technology for studying spectra. Engineers in turn looked toward the sciences in other fields, such as optics and imaging, and now we have technology like the Sloan Digital Sky Survey, which takes 1000 spectrograms simultaneously. Not bad for progress.

Scientific Knowledge Assumes an Order and Consistency in Natural Systems: Scientific knowledge is based on the assumption that natural laws operate today as they did in the past and they will continue to do so in the future.

Your students should understand that while scientists are still learning the nuances of how the universe works, we assume the universe still goes about following the same natural laws as it has for billions of years. After all, there's no reason for it to have decided to start following a different set of rules.

With that assumption, we can study our sun, for example, and assume that the process of fusion taking place in the core is happening in the same way that happened in stars that died billions of years ago.

Scientific Knowledge Assumes an Order and Consistency in Natural Systems: Science assumes the universe is a vast single system in which basic laws are consistent.

Students should know that in the same way that natural laws stay consistent over time, we assume that they are consistent everywhere in the entire universe, whether we're right here in the comfort of our own solar system or a couple thousand parsecs away.

This is important, because it allows us to study with confidence things like emission spectra. We have no way of knowing with 100% confidence what a star thousands of light years away is made out of, but we do know the emission spectrum of every element here in our neck of the universe. That means we can use that knowledge to identify the spectra of faraway places.