Can You Ace Harvard's Galactic Quiz?

[Hornbein]

https://news.harvard.edu/gazette/story/2025/05/strange-galactic-facts/

Harvard's quiz. I got a disappointing 50%.

 

[berkeman]

Clicking into the quiz, the first question is badly posed. Is it worth my time to proceed? I think not.

 

[Jaime Rudas]

On the second question, I answered 21%, and the quiz corrected me for 5%. I still think it's around 21%. What do you think?

 

[Jaime Rudas]

Clicking into the quiz, the first question is badly posed. Is it worth my time to proceed? I think not.

It seems to me that it could be misinterpreted, but it's not badly posed. In fact, I think the proposed answer (over a trillion) is correct.

 

[PeterDonis]

I think the proposed answer (over a trillion) is correct.

Since the universe is spatially infinite in our best current model, the correct answer would be "an infinite number". Technically this is "over a trillion", but the question strongly implies that the correct answer is a finite number.

Unless the question is meant to refer only to our observable universe--but it doesn't say so. So I have to agree with @berkeman that the question is badly posed.

 

[PeterDonis]

I still think it's around 21%.

Based on what reference?

 

[Bandersnatch]

Based on what reference?

Do you really need one? It's obviously about the fraction of baryonic to dark matter.
On which point, I also don't see how it makes sense to include dark energy in this context.

 

[Jaime Rudas]

Since the universe is spatially infinite in our best current model, the correct answer would be "an infinite number".

According to the most precise measurements available (Planck 2018), we cannot rule out that the universe is finite, but we can rule out that the universe has fewer than a trillion galaxies.

 

[pinball1970]

https://news.harvard.edu/gazette/story/2025/05/strange-galactic-facts/

Harvard's quiz. I got a disappointing 50%.

I thought I was going to smash it after the first 5 but then fell off a cliff. 60%

The ones I got wrong

Measuring galaxy size
Where are the oldest stars in the Milky way
Oddity about Lenticular galaxies
Last activity of Sagittarius A black hole

 

[Jaime Rudas]

Do you really need one? It's obviously about the fraction of baryonic to dark matter.
On which point, I also don't see how it makes sense to include dark energy in this context.

That's right. The density of galaxies is many orders of magnitude greater than the density of the universe, so the density of dark energy (which is constant) is negligible in this context.

 

[sbrothy]

Surprisingly, I got 60% but I'm afraid it was mostly guesswork.

 

[PeterDonis]

Do you really need one?

Um, yes, to back up the specific value that was claimed.

 

[Bosko]

Surprisingly, I got 60% but I'm afraid it was mostly guesswork.

The same

 

[Jaime Rudas]

Um, yes, to back up the specific value that was claimed.

In this type of quiz, it's normal and expected that one of the proposed options will correspond to the correct answer. Thus, the choice of this "specific value" is based on the fact that, for the reasons explained in post #8, it's the one I consider most likely to be correct.

 

[PeterDonis]

the reasons explained in post #8

That post was about why you think the "to a trillion or more" answer to the "how many galaxies" question was correct. I was asking you about why you think the 21% figure for how much of a typical galaxy's mass is ordinary matter is correct.

 

[Bandersnatch]

@PeterDonis From Planck 2018 results we get density of dark matter at 0.27, baryonic at 0.05. That gives a ratio of about 18%, which is the closest to the choice provided.
But you know this, and its hardly news to anyone, that would require referencing. So could you be a bit clearer about what you're objecting to? Is it that 18 is not 21?

 

[javisot]

@PeterDonis From Planck 2018 results we get density of dark matter at 0.27, baryonic at 0.05. That gives a ratio of about 18%, which is the closest to the choice provided.
But you know this, and its hardly news to anyone, that would require referencing. So could you be a bit clearer about what you're objecting to? Is it that 18 is not 21?

I understand that in this case Peter Donis's request for references is simply curiosity to know the origin of that 21% that Jaime supposes.

 

[Jaime Rudas]

That post was about why you think the "to a trillion or more" answer to the "how many galaxies" question was correct. I was asking you about why you think the 21% figure for how much of a typical galaxy's mass is ordinary matter is correct.

Yes, you are right: it's actually for the reasons explained in post #10.

 

[Jaime Rudas]

I understand that in this case Peter Donis's request for references is simply curiosity to know the origin of that 21% that Jaime supposes.

Yes, it's possible, but the point is that the "specific value" 21 wasn't suggested by me; it corresponds to the option provided in the quiz that, in my opinion, best fits the question given.

 

[Jaime Rudas]

Based on what reference?

Based on the data in this paper, we can deduce that the baryonic mass corresponds to about 33% of the total mass of the Milky Way.

 

[PeterDonis]

That gives a ratio of about 18%, which is the closest to the choice provided.

Based on the data in this paper, we can deduce that the baryonic mass corresponds to about 33% of the total mass of the Milky Way.

Yes, and neither of those are 21%, and the quiz did not give a range for that choice--unlike the "less than 5 percent" of the first choice. So I'm wondering if there is a reference that actually estimated it at 21%.

 

[javisot]

We're talking averages, that is, "on average, how much of a typical galaxy's mass is ordinary matter."

Jaime is closer to 21% because he has in mind the fact that in the case of the Milky Way, it's 33%. But the average across all galaxies comes to a mere 5% or less, according to this questionnaire.

 

[sbrothy]

We're talking averages, that is, "on average, how much of a typical galaxy's mass is ordinary matter."

Jaime is closer to 21% because he has in mind the fact that in the case of the Milky Way, it's 33%. But the average across all galaxies comes to a mere 5% or less, according to this questionnaire.

That was one of the questions I "guessed" "correctly".

 

[Jaime Rudas]

Yes, and neither of those are 21%, and the quiz did not give a range for that choice--unlike the "less than 5 percent" of the first choice. So I'm wondering if there is a reference that actually estimated it at 21%.

Yes, that's precisely why I indicated that I thought it was AROUND 21% and not the "less than 5%" that the quiz indicates.

 

[PeterDonis]

I thought it was AROUND 21% and not the "less than 5%" that the quiz indicates.

Yes, but the quiz, as I said, doesn't say "around 21%" or give a range for that choice, whereas it does for the first choice. That seems odd to me.

 

[Bandersnatch]

It's not that they're giving a range for the desired answer, imo, as they're rounding the number. The value for omega b you'll find in the Planck results is 4.9 and change. So a bit less than 5. They don't care as much for the answers they think are wrong, and probably assigned the numbers willy-nilly.

 

[javisot]

Yes, but the quiz, as I said, doesn't say "around 21%" or give a range for that choice, whereas it does for the first choice. That seems odd to me.

They also do a similar thing with the question about the number of galaxies in the universe. The first three options assume the universe is finite, but option 4 "1 trillion or more" is a range that spans from 1 trillion to infinity.

For those two questions, it would be strange if the answer were an exact percentage, without a range (as if we knew the answer with arbitrary precision, which is not the case)

 

[Beyond3D]

https://news.harvard.edu/gazette/story/2025/05/strange-galactic-facts/

Harvard's quiz. I got a disappointing 50%.

I beat you and I am a teenager lol. Pretty apt as I plan to minor in astronomy in university and major in physics.