In addition to conventional galaxies, the universe contains very dim

Question

In addition to conventional galaxies, the universe contains very dim galaxies that until recently went unnoticed by astronomers. Possibly as numerous as conventional galaxies, these galaxies have the same general shape and even the same approximate number of stars as a common type of conventional galaxy, the spiral, but tend to be much larger. Because these galaxies’ mass is spread out over larger areas, they have far fewer stars per unit volume than do conventional galaxies. Apparently these low-surface-brightness galaxies, as they are called, take much longer than conventional galaxies to condense their primordial gas and convert it to stars—that is, they evolve much more slowly.

These galaxies may constitute an answer to the  long-standing puzzle  of the missing baryonic mass in the universe. Baryons—subatomic particles that are generally protons or neutrons—are the source of stellar, and therefore galactic, luminosity, and so their numbers can be estimated based on how luminous galaxies are. However, the amount of helium in the universe, as measured by spectroscopy, suggests that there are far more baryons in the universe than estimates based on galactic luminosity indicate. Astronomers have long speculated that the missing baryonic mass might eventually be discovered in intergalactic space or as some large population of galaxies that are difficult to detect.  
 1 
 91. According to the passage, conventional spiral galaxies differ from low-surface-brightness galaxies in which of the following ways?

(A) They have fewer stars than do low-surface brightness galaxies.
(B) They evolve more quickly than low-surfacebrightness galaxies.
(C) They are more diffuse than low-surface brightness galaxies.
(D) They contain less helium than do low-surface brightness galaxies.
(E) They are larger than low-surface-brightness galaxies.

2 
 92. It can be inferred from the passage that which of the following is an accurate physical description of typical low-surface-brightness galaxies?

(A) They are large spiral galaxies containing fewer stars than do conventional galaxies.
(B) They are compact but very dim spiral galaxies.
(C) They are diffuse spiral galaxies that occupy a large volume of space.
(D) They are small, young spiral galaxies that contain a high proportion of primordial gas.
(E) They are large, dense spirals with low luminosity.

3 
 93. It can be inferred from the passage that the “longstanding puzzle” refers to which of the following?

(A) The difference between the rate at which conventional galaxies evolve and the rate at which low-surface-brightness galaxies evolve
(B) The discrepancy between estimates of total baryonic mass derived from measuring helium and estimates based on measuring galactic luminosity
(C) The inconsistency between the observed amount of helium in the universe and the number of stars in typical low-surface-brightness galaxies
(D) Uncertainties regarding what proportion of baryonic mass is contained in intergalactic space and what proportion in conventional galaxies
(E) Difficulties involved in detecting very distant galaxies and in investigating their luminosity

4 
 94. The author implies that low-surface-brightness galaxies could constitute an answer to the puzzle discussed in the second paragraph primarily because

(A) they contain baryonic mass that was not taken into account by researchers using galactic luminosity to estimate the number of baryons in the universe
(B) they, like conventional galaxies that contain many baryons, have evolved from massive, primordial gas clouds
(C) they may contain relatively more helium, and hence more baryons, than do galaxies whose helium content has been studied using spectroscopy
(D) they have recently been discovered to contain more baryonic mass than scientists had thought when low-surface-brightness galaxies were first observed
(E) they contain stars that are significantly more luminous than would have been predicted on the basis of initial studies of luminosity in lowsurface-brightness galaxies

5 
 95. The author mentions the fact that baryons are the source of stars’ luminosity primarily in order to explain

(A) how astronomers determine that some galaxies contain fewer stars per unit volume than do others
(B) how astronomers are able to calculate the total luminosity of a galaxy
(C) why astronomers can use galactic luminosity to estimate baryonic mass
(D) why astronomers’ estimates of baryonic mass based on galactic luminosity are more reliable than those based on spectroscopic studies of helium
(E) how astronomers know bright galaxies contain more baryons than do dim galaxies

6 
 96. The author of the passage would be most likely to disagree with which of the following statements?

(A) Low-surface-brightness galaxies are more difficult to detect than are conventional galaxies.
(B) Low-surface-brightness galaxies are often spiral in shape. 
(C) Astronomers have advanced plausible ideas about where missing baryonic mass might be found.
(D) Astronomers have devised a useful way of estimating the total baryonic mass in the universe.
(E) Astronomers have discovered a substantial amount of baryonic mass in intergalactic space.

7 
 97. The primary purpose of the passage is to

(A) describe a phenomenon and consider its scientific significance
(B) contrast two phenomena and discuss a puzzling difference between them
(C) identify a newly discovered phenomenon and explain its origins
(D) compare two classes of objects and discuss the physical properties of each
(E) discuss a discovery and point out its inconsistency with existing theory

Gmatsaiyan · Accepted Answer

Official Answers and Explanations 1. According to the passage, conventional spiral galaxies differ from low-surfacebrightness
galaxies in which of the following ways?
A. They have fewer stars than do low-surface-brightness galaxies.
B. They evolve more quickly than low-surface-brightness galaxies.
C. They are more diffuse than low-surface-brightness galaxies.
D. They contain less helium than do low-surface-brightness galaxies.
E. They are larger than low-surface-brightness galaxies.
 Supporting ideas 
This question requires recognizing information that is provided in the passage.
The first paragraph describes and compares two types of galaxies: conventional galaxies and dim, or low-surface-brightness, galaxies. It states that dim galaxies have the same approximate number of stars as a common type of conventional galaxy but tend to be larger and more diffuse because their mass is spread over wider areas . The passage also indicates that dim galaxies take longer than conventional galaxies to convert their primordial gases into stars, meaning that dim galaxies evolve much more slowly than conventional galaxies , which entails that conventional galaxies evolve more quickly than dim galaxies.
A. The passage states that dim galaxies have approximately the same numbers of stars as a common type of conventional galaxy.
B. Correct. The passage indicates that dim galaxies evolve much more slowly than conventional galaxies, which entails that conventional galaxies evolve more quickly.
C. The passage states that dim galaxies are more spread out, and therefore more diffuse, than conventional galaxies.
D. The passage does not mention the relative amounts of helium in the two types of galaxies under discussion.
E. The passage states that dim galaxies tend to be much larger than conventional galaxies.
The correct answer is B.

2. It can be inferred from the passage that which of the following is an accurate physical description of typical low-surface-brightness galaxies?
A. They are large spiral galaxies containing fewer stars than conventional galaxies.
B. They are compact but very dim spiral galaxies.
C. They are diffuse spiral galaxies that occupy a large volume of space.
D. They are small, young spiral galaxies that contain a high proportion of primordial gas.
E. They are large, dense spirals with low luminosity.
 Inference 
This question requires drawing an inference from information given in the passage. The first paragraph compares dim galaxies and conventional galaxies. Dim galaxies are described as having the same general shape (lines 4–5) as a common type of conventional galaxy, the spiral galaxy, suggesting that dim galaxies are, themselves, spiral shaped. The passage also indicates that, although
both types of galaxies tend to have approximately the same number of stars, dim galaxies tend to be much larger and spread out over larger areas of space than conventional galaxies.
A. The passage states that the two types of galaxies have approximately the same number of stars.
B. The passage indicates that dim galaxies are relatively large and spread out.
C. Correct. The passage indicates that dim galaxies have the same general shape as spiral galaxies and that their mass is spread out over large areas of space.
D. The passage indicates that dim galaxies are relatively large and spread out.
E. The passage states that dim galaxies have few stars per unit of volume,
suggesting that they are not dense but diffuse.
The correct answer is C.

3. It can be inferred from the passage that the “long-standing puzzle” refers to which of the following?
A. The difference between the rate at which conventional galaxies evolve and the rate at which low-surface-brightness galaxies evolve
B. The discrepancy between estimates of total baryonic mass derived from measuring helium and estimates based on measuring galactic luminosity
C. The inconsistency between the observed amount of helium in the universe and the number of stars in typical low-surface-brightness galaxiesD. Uncertainties regarding what proportion of baryonic mass is contained in intergalactic space and what proportion in conventional galaxies
E. Difficulties involved in detecting very distant galaxies and in investigating their luminosity
 Inference 
This question requires drawing an inference from information given in the passage. The second paragraph describes the long-standing puzzle of the missing baryonic mass in the universe. The passage states that baryons are the source of galactic luminosity, and so scientists can estimate the amount of baryonic mass in the universe by measuring the luminosity of galaxies (lines 17–21). The puzzle is that spectroscopic measures of helium in the universe suggest that the baryonic mass in the universe is much higher than measures of luminosity would indicate.
A. The differences between the rates of evolution of the two types of galaxies is not treated as being controversial in the passage.
B. Correct. The passage indicates that measurements using spectroscopy and measurements using luminosity result in puzzling differences in estimates of the universe’s baryonic mass.
C. The passage does not suggest how helium might relate to the numbers of stars in dim galaxies.
D. The passage indicates that astronomers have speculated that the missing baryonic mass might be discovered in intergalactic space or hard-to-detect galaxies but does not suggest that these speculations are constituents of the long-standing puzzle.
E. The passage does not mention how the distance to galaxies affects scientists’ ability to detect these galaxies.
The correct answer is B.

4. The author implies that low-surface-brightness galaxies could constitute an answer to the puzzle discussed in the second paragraph primarily because
A. they contain baryonic mass that was not taken into account by researchers using galactic luminosity to estimate the number of baryons in the universe
B. they, like conventional galaxies that contain many baryons, have evolved from massive, primordial gas clouds
C. they may contain relatively more helium, and hence more baryons, than do galaxies whose helium content has been studied using spectroscopy
D. they have recently been discovered to contain more baryonic mass than scientists had thought when low-surface-brightness galaxies were firstobserved
E. they contain stars that are significantly more luminous than would have been predicted on the basis of initial studies of luminosity in low-surfacebrightness galaxies
 Inference 
This question requires drawing an inference from information given in the passage. The puzzle is that estimates of the baryonic mass of the universe based on luminosity are lower than those based on spectroscopy . The passage states that astronomers did not notice dim galaxies until recently and that these galaxies may help account for the missing baryonic mass in the universe . The passage also suggests that astronomers measure the luminosity of specific galaxies . Thus it can be inferred that, prior to their being noticed by astronomers, the luminosity of these dim galaxies was not measured, and their baryonic mass was not taken into account in the estimates of luminosity that led to the long-standing puzzle.
A. Correct. The passage states that the missing baryonic mass in the universe may be discovered in the dim galaxies that have only recently been noticed by astronomers.
B. The passage does not suggest that dim and conventional galaxies both originating from primordial gas clouds help solve the long-standing puzzle of the missing baryonic mass in the universe.
C. The passage does not suggest that dim galaxies might contain more helium than do conventional galaxies or that measures of baryonic mass using spectroscopy do not take some dim galaxies into account.
D. The passage does not suggest that dim galaxies contain more baryonic mass than scientists originally believed upon discovering these galaxies.
E. The passage suggests that scientists measured the luminosity of galaxies, not of individual stars.
The correct answer is A.

5. The author mentions the fact that baryons are the source of stars’ luminosity primarily in order to explain
A. how astronomers determine that some galaxies contain fewer stars per unit volume than do others
B. how astronomers are able to calculate the total luminosity of a galaxy
C. why astronomers can use galactic luminosity to estimate baryonic mass
D. why astronomers’ estimates of baryonic mass based on galactic luminosity are more reliable than those based on spectroscopic studies of heliumE. how astronomers know bright galaxies contain more baryons than do dim galaxies
 Evaluation 
This question requires understanding how one aspect of the passage relates to the reasoning in a larger portion of the passage. The second paragraph explains that scientists have been puzzled over missing baryonic mass in the universe as measured by luminosity (lines 21–25). Given that baryons are the source of luminosity in the galaxy (lines 17–19), astronomers can estimate the baryonic mass of a galaxy by measuring its luminosity.
A. The passage discussion of baryons does not address the number of stars in individual galaxies.
B. The passage discusses how the luminosity of galaxies can be used to estimate baryonic mass but does not address how total luminosity is measured.
C. Correct. The passage indicates that because baryons are the source of galactic luminosity, measuring luminosity can be used to estimate baryonic
mass of galaxies.
D. The passage suggests that estimates based on luminosity may have been less accurate, not more accurate, than those based on spectroscopy.
E. The passage does not indicate that bright galaxies contain more baryons than do dim galaxies.
The correct answer is C.

6. The author of the passage would be most likely to disagree with which of the following statements? 
A. Low-surface-brightness galaxies are more difficult to detect than are conventional galaxies.
B. Low-surface-brightness galaxies are often spiral in shape.
C. Astronomers have advanced plausible ideas about where missing baryonic mass might be found.
D. Astronomers have devised a useful way of estimating the total baryonic mass in the universe.
E. Astronomers have discovered a substantial amount of baryonic mass in intergalactic space.
 Inference 
This question involves identifying which answer option potentially conflicts with the information the author has provided in the passage. The second paragraphindicates that astronomers’ estimates of the baryonic mass of the universe is lower when measured using luminosity than it is when measured using spectroscopy . The final sentence states that astronomers have speculated that the missing baryonic mass might be discovered in intergalactic space or in hard-to-detect galaxies (lines 25–29). Although the passage does indicate that the discovery of dim, low-surface-brightness galaxies might help account for the missing baryonic mass (lines 15–17), the passage provides no support for the possibility that baryonic mass has been discovered in intergalactic space.
A. The passage indicates that low-surface-brightness galaxies went unnoticed until recently, unlike conventional galaxies.
B. The passage indicates that low-surface-brightness galaxies have the same general shape as spiral galaxies.
C. The passage describes two possible explanations astronomers have given for the missing baryonic mass, one of which was made more plausible by the discovery of low-surface-brightness galaxies. 
D. The passage indicates that astronomers have used spectroscopy to estimate baryonic mass and gives no reason to suspect that this method is not useful.
E. Correct. The passage does not indicate that astronomers have found any baryonic mass in intergalactic space.
The correct answer is E.

7. The primary purpose of the passage is to
A. describe a phenomenon and consider its scientific significance
B. contrast two phenomena and discuss a puzzling difference between them
C. identify a newly discovered phenomenon and explain its origins
D. compare two classes of objects and discuss the physical properties of each
E. discuss a discovery and point out its inconsistency with existing theory
 Main idea 
This question requires understanding, in broad terms, the purpose of the passage as a whole. The first paragraph describes a phenomenon: the discovery of dim galaxies and some of their general attributes. The second paragraph describes how this discovery may help astronomers to solve a long-standing puzzle about the baryonic mass of the universe.
A. Correct. The passage describes the phenomenon of dim galaxies and describes their significance in solving the long-standing puzzle of the missing baryonic mass in the universe.
B. Although the passage discusses the puzzling difference between the two estimates of baryonic mass, this option does not account for the broader topic of dim galaxies.
C. While the passage identifies the newly discovered phenomenon of dim galaxies, it does not offer a significant explanation for these galaxies’ origins.
D. Although the passage compares dim and conventional galaxies in the first paragraph, this option does not account for the important detail that dim galaxies may help solve a long-standing puzzle.
E. The discovery of dim galaxies discussed in the passage is not said to be inconsistent with any existing scientific theory.

kivalo · Answer

Can someone explain how do you read a passage like this one (relatively clear structure, but a lot of supporting details to comprehend). I got 7/7 right in 10 mins, but still feel that I haven't built a consistent approach. Can someone who is good at this explain (as I do below) how to do it right, i.e. how to know how much time and attention to develop to specific details ? What do you exactly take from your first read?

Most important sentence. There are two type of galaxies, conventional and dim. Until recently, scientists did not know about the dim galaxies.
 
 This is the part I am not sure about. I'd note only that bunch of differences are given, and skim to try to comprehend what the differences are. During my read, I happened to remember the difference in size in density, but what I remembered was not important

Again, I am not sure how to tackle this para. 
  Again, here I'd only note that knowledge about dim galaxies might help explain why measures relying on luminosity and helium give different estimates of baryonic mass in the universe  However, the paragraph contains a lot more info that requires time to comprehend. What do I do with all that information during the  first read?

AD2GMAT · Answer

Though I am not an expert, I say identifying the structure in the first read is good enough to go ahead with the questions. In other words the "minimum" amount of reading that is necessary for you to answer the Primary Purpose question is enough in the first reading. 
If some expert reply to this question, then I suppose it would be more fruitful for you and for me also.

Skywalker18 · Answer

Decent passage , got all correct except Q96(6th question) in 12 mins , including 3 mins 15 seconds to read .
For Q96 , i was stuck between option D and E and could not eliminate either .

AjiteshArun  ,  mikemcgarry  , GMATNinja  ,  ccooley  , other experts -- can you please help ?

AjiteshArun · Answer

The passage says  astronomers have long speculated that  the missing baryonic mass might eventually be discovered in intergalactic space  , but option E says  astronomers  have discovered a substantial amount of baryonic mass in intergalactic space  .

According to the passage, the mass might be discovered (in the future). Option E says that the discovery has already happened. Therefore, we'd expect the author to disagree with E.

aragonn · Answer

P1- new galaxies; slow; dim; diff with conventional galaxies.
P2 - how it is a missing puzzle. baryons
Main point - about new galaxies - similarity with conventional one and importance of their discovery.

91. According to the passage, conventional spiral galaxies differ from low-surface-brightness galaxies in which of the following ways?
must be in P1

(A) They have fewer stars than do low-surface brightness galaxies. - not true for all
(B) They evolve more quickly than low-surfacebrightness galaxies. - this must be correct.
(C) They are more diffuse than low-surfacebrightness galaxies. - not true for all
(D) They contain less helium than do low-surfacebrightness galaxies. - not true for all
(E) They are larger than low-surface-brightness galaxies. - not true for all
----------------------------------------

92. It can be inferred from the passage that which of the following is an accurate physical description of typical low-surface-brightness galaxies?

(A) They are large spiral galaxies containing fewer stars than do conventional galaxies. - no; the same approximate number of stars
(B) They are compact but very dim spiral galaxies. - dim is true but compact is not. 
(C) They are diffuse spiral galaxies that occupy a large volume of space. - has to be this; the same approximate number of stars as a common type of conventional galaxy, the spiral, but tend to be much larger
(D) They are small, young spiral galaxies that contain a high proportion of primordial gas. - too broad.
(E) They are large, dense spirals with low luminosity. - no dense

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93. It can be inferred from the passage that the “longstanding puzzle” refers to which of the following?
anchor - longstanding puzzle - of the missing baryonic mass in the universe ---- Astronomers have long speculated that the missing baryonic mass might eventually be discovered in intergalactic space or as some large population of galaxies that are difficult to detect.

(B) The discrepancy between estimates of total baryonic mass derived from measuring helium and estimates based on measuring galactic luminosity - correct; best of the lot.

---------------------------------------
94. The author implies that low-surface-brightness galaxies could constitute an answer to the puzzle discussed in the second paragraph primarily because

the missing baryonic mass might eventually be discovered in intergalactic space or as some large population of galaxies that are difficult to detect.

(A) they contain baryonic mass that was not taken into account by researchers using galactic luminosity to estimate the number of baryons in the universe -

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95. The author mentions the fact that baryons are the source of stars’ luminosity primarily in order to explain

(C) why astronomers can use galactic luminosity to estimate baryonic mass - yes

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96. The author of the passage would be most likely to disagree with which of the following statements?

(A) Low-surface-brightness galaxies are more difficult to detect than are conventional galaxies. - T
(B) Low-surface-brightness galaxies are often spiral in shape. - T
(C) Astronomers have advanced plausible ideas about where missing baryonic mass might be found. - T
(D) Astronomers have devised a useful way of estimating the total baryonic mass in the universe. - T
(E) Astronomers have discovered a substantial amount of baryonic mass in intergalactic space. - only theory is given here.

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97. The primary purpose of the passage is to

(A) describe a phenomenon and consider its scientific significance - to the point. P2 defines scientific significance.
(B) contrast two phenomena and discuss a puzzling difference between them - too weak
(C) identify a newly discovered phenomenon and explain its origins - explain its origins ---- seems wrong
(D) compare two classes of objects and discuss the physical properties of each - P1 only
(E) discuss a discovery and point out its inconsistency with existing theory - inconsistency ---- not true

Chelsea212 · Answer

This was a tough one! I unexpectedly found myself struggling with RC after having zero problems doing Manhattan practice exams. When I did the Manhattan Prep test, I can't even recall getting 1 RC question wrong ever...but then when I transitioned to the OG stuff, I found that I was really struggling. That said, I always, always,  always  suggest that people focus on using reading materials from the science/technology, as well as global business sections from The Economist and the NYTimes as practice, and then testing for improvement by practicing only OG questions. In my personal experience, this has yielded the largest improvement for me.

I think a key part of my improvement thus far has also hinged on the fact that I deliberately pick articles I just do not want to read because (a) the topic is too foreign and (b) the topic seems super boring. Articles about galaxies, stars, and general science typically bring on the ZZzzZs for me so I pick those as they are both boring and incredibly confusing to me.

One of the most basic points you should be able to answer after every practice article is "what was the point of this article?" and being able to articulate it in a very short but succinct sentence. If you're going to keep this question in mind, you'll intuitively find yourself transitioning from reading 'fact, fact, fact' to a sentence by sentence structural understanding of 'introduction, opinion, opposing opinion, explanation etc'. I highly recommend that you read/watch videos done by the GMATNinjas and Mike McGarry if you want a refresher on how to read.

Now on with the RC! I'm going to start with commenting on the final question because it is equally basic as it is difficult question.

Step 1. I always, always, always rephrase the passage as I read.  - This passage is fairly short, and there isn't much we need to do to organize our thoughts because the writer seems to have done the bulk of it. The first paragraph introduces the topic, comparing and contrasting two types of galaxies. The second paragraph moves into how the 'dim galaxies' may hold answers to a long-standing puzzle.

Step 2. For every question, always go back to the passage.  - When I reach this question, I quickly revisit the passage at a very high level to again paraphrase the highlights of the passage. This jogs my memory so that I can get a good handle on what I'm going to be looking out for in the answer choices of a high-level, big picture type of question (i.e. not detail related).

Step 3. Always find 4 wrong answers! 
 
(A) describe a phenomenon and consider its scientific significance  - This sounds promising...not a  bingo!  straight away in my mind, but I will definitely put this guy on hold.  I've returned after eliminating all else, and this now sounds spot on!

(B) contrast two phenomena and discuss a puzzling difference between them  - The passage compared two types of a single phenomena: galaxies. Although I'm not certain, I don't think I would call the conventional and dim galaxies two separate phenomena, but rather, 2 types of a single phenomena. Also, the primary purpose was not discussing a puzzling difference between these two galaxies. I think the primary purpose was captured in par-graph 2 sentence 1: "These galaxies may constitute an answer to the long-standing puzzle of the missing baryonic mass in the universe." The dim galaxies seem to just have some sort of scientific significance in resolving a long unsolved puzzle.

(C) identify a newly discovered phenomenon and explain its origins  - There is no mention of any 'newly' discovered phenomenon, but rather, a 'long-standing puzzle'.. In addition, the passage never touches on, let alone explains, origins of any phenomena.

(D) compare two classes of objects and discuss the physical properties of each  - Nice, this answer choice gives me more confidence in my justification for answer choice (B). While the first paragraph does exactly what this answer choice states, it is not the primary purpose of the passage. As we can see, the first sentence of the second paragraph introduces the purpose of the passage as it effectively switches the tone of the passage from being explanatory to questioning while using the information from paragraph as context.

The second paragraph is where the heart of the passage's purpose lies and the first and last sentence of the second paragraph structurally cement this. For the purposes of eliminating this specific answer choice, I think it is enough to recognize that there is a second paragraph and it is  not  at all doing what this choice states, but instead opening up room for discussion and further exploration.

(E) discuss a discovery and point out its inconsistency with existing theory  - I think that this is a very tempting answer choice because it is very abstract. I do not think I give the best explanation as to how and why I eliminate this choice, but I'll do my best! I actually got this question wrong and chose this after I got down to choosing between (A) and (E) and was feeling like I needed to just pick!

First point of confusion for me here is that I am uncertain whether I would label these galaxies being discussed as discoveries in present day...they were definitely 'discovered' and i.e. new phenomena at some point.... Even if I call these galaxies a new discovery, the passage does not point out their inconsistency with any existing theory. Instead, the passage discusses the inconsistency on the amount of Baryons are in one of them as measured by two different methods.

I think that if the purpose of the passage was to discuss a 'discovery', the author would very deliberately mention the word discovery. On this basis, I eliminate this choice.

Main takeaway:  Answer choices like (E) are prime examples of abstract thoughts that touch enough on relevant and true portion so the passage that it is a very easy trap to fall into.

Feedback and comment not only welcome, but encouraged!! I think lots of people would benefit from discussion on this one.

psls · Answer

How many galaxies ? i got confused by the "these galaxies" is this referring to conventional or dim galaxies?

It is safe to say all the pronouns used are referring to the subject?

(Because these galaxies’ mass is spread out over larger areas, they have far fewer stars per unit volume than do conventional galaxies.)

"Because these galaxies" Is this referring to dim galaxies?

(Apparently these low-surface-brightness galaxies, as they are called, take much longer than conventional galaxies to condense their primordial gas and convert it to stars—that is, they evolve much more slowly.)

"These low-surface-brightness galaxies": is 3rd galaxies? not Dim or conventional galaxies?

(These galaxies may constitute an answer to the long-standing puzzle of the missing baryonic mass in the universe.)

Is the "These galaxies" here refer to dim galaxies too?

GMATNinja · Answer

You're right, "these galaxies" is referring to the subject of the first sentence, the  dim galaxies .

This is true throughout this passage -- each time "these galaxies" is used, it's referring to the  dim galaxies  mentioned in the first sentence.

The "low-surface-brightness galaxies" are also the  dim galaxies , this part of the passage just gives another name for them. There are only two types of galaxy in this passage: the conventional galaxies and the dim galaxies.

I hope that helps!

psls · Answer

Dear Experts,

GMATNinja 
 VeritasKarishma 
 AjiteshArun

In Q6., I got hesitated in choice B,

According to the passage,

Why author would agree with choice(B) with the word  "OFTEN" ?
Since I cannot see any mentions of the likelihood of galaxy shapes.

How can I eliminate it with confidence?

Appreciate,
-tor

AjiteshArun · Answer

Hi  TorGmatGod ,

The correct answer will be something that the author is (most) likely to disagree with. In the portion of the passage you're looking at, the author clearly says "these galaxies have  the same general shape... as  a common type of conventional galaxy,  the spiral ". This means that  ~   low-surface-brightness galaxies are spiral in shape . The only difference between what we see in the passage and what option B says is that option B includes the word  often .

(1) Low-surface-brightness galaxies are spiral in shape. ← This is what the passage tells us.
(2) Low-surface-brightness galaxies are  often  spiral in shape. ← This is option B.

Someone who agrees with (1) will most likely also agree with (2). If it helps, we can read B as "many low-surface-brightness galaxies are spiral in shape". If all low-surface-brightness galaxies are spiral in shape, then the statement "many low-surface-brightness galaxies are spiral in shape" is also true, so the author is unlikely to disagree with it. On the other hand, the author would definitely disagree with E, because he or she says that the missing baryonic mass "might eventually be discovered in intergalactic space".

KarishmaB · Answer

"same general shape" and "often have the same shape" are not very different. 
"Same general shape" means that most have this shape but there could be some exceptions. "Often have the same shape" is also the same idea. When you are talking about galaxies, there is a lot we don't know. So making definitive statements is not possible.

TheUltimateWinner · Answer

AjiteshArun 
Thanks for the nice explanation, but it confused a bit me.
 
It seems that  Low-surface-brightness galaxies  is same as  dim galaxies , right? From the above quote, the  conventional galaxy  is  the spiral  as  the spiral  has been used as the modifier of  conventional galaxy . So, from the quote, it seems that  conventional galaxy  (not Low-surface-brightness galaxies/ dim galaxies) are spiral in shape.
Am I missing anything here?
Thank you very much.

AjiteshArun · Answer

Hi  TheUltimateWinner ,

Let's look at that part of the passage again:
 
This is where the author tells us that very dim galaxies have the same general shape as spiral galaxies do.

Sneha2021 · Answer

Hi Experts,
 KarishmaB   GMATNinja

I was confused btw D & E in Q96.
96. The author of the passage would be most likely to disagree with which of the following statements? 
(D) Astronomers have devised a useful way of estimating the total baryonic mass in the universe.
(E) Astronomers have discovered a substantial amount of baryonic mass in intergalactic space.

"Astronomers have long speculated that the missing baryonic mass might eventually be discovered in intergalactic space or as some large population of galaxies that are difficult to detect"
From these 2 lines, we can infer that Astronomers might discover baryonic mass in future. So author doesn't completely disagree with the idea that baryonic mass can be discovered or would have been discovered. Author has shared hints about the discovery in a positive way.
The reason I rejected D bcz author didn't share any perspective on devising a useful way of estimating baryonic mass. We don't know author's perspective regarding this new way.

95. The author mentions the fact that baryons are the source of stars’ luminosity primarily in order to explain
(E) how astronomers know bright galaxies contain more baryons than do dim galaxies
Why E is incorrect?

Based on luminosity, we can calculate no of baryons. So astronomers can say bright galaxies will have more no. of baryons compared to dim. Why E is incorrect then?

Thank you very much.

AjiteshArun · Answer

Hi  Sneha2021 ,

Let's take a look at some of the important words in that part of the passage.
 
Option E tells us that astronomers  have (already) discovered  a  substantial amount  of baryonic mass  in intergalactic space . If they've already discovered so much baryonic mass in intergalactic space, why are they still speculating that intergalactic space would be a good place to go looking for the missing baryonic mass? The passage certainly doesn't tell us that they've already found lots of baryonic mass there and now want to look for even more, so it's more likely that they haven't found any (or much) baryonic mass in intergalactic space yet, and that's why they think intergalactic space may where the missing baryonic mass is. Even the second candidate ("some large population of galaxies that are difficult to detect") is something that we wouldn't expect astronomers to already know a great deal about.

GMATNinja · Answer

Question 6 (or 96) 
 
Starting with (E):
 
The words "have discovered" very specifically tell us that astronomers  already discovered  a substantial amount of baryonic mass in intergalactic space. The last line of the passage tells us that astronomers have  speculated  that this mass exists, but nothing indicates that it has already been discovered, as written in (E).

Additionally, the same line discusses  two  possible answers to the long-standing puzzle of extra mass: matter in intergalactic space, or extra galaxies that are difficult to detect.

The passage provides ample support for the second answer: these difficult-to-detect galaxies HAVE been detected. They may well account for the entirety of the missing mass. By contrast, the author doesn't provide any evidence to support the idea that there is a substantial amount of baryonic mass in intergalactic space.

The author would disagree that astronomers have discovered a substantial amount of baryonic mass in intergalactic space, so (E) is the correct answer.

Here's (D):
 
The author discusses two ways of estimating baryonic mass: luminosity, and "the amount of helium in the universe, as measured by spectroscopy." This second method is used to estimate the total baryonic mass in the universe, so that's the method (D) is talking about.

There is a gap between the two measures -- the measured luminosity of galaxies is low compared to measured amount of helium. From this, the author says that there is "missing" matter. This means that the author  trusts  the higher number given by the helium method, and is searching for a reason to explain why the luminosity is lower than it should be.

Because the author trusts the helium method of measuring the total baryonic mass in the universe, the author would agree with (D).

(D) is out.

Question 5 (or 95) 
 
Here's the exact wording of question 5:
 
To answer this question, we're not just looking for something that's true about the "fact." We're looking for the  primary reason WHY  the author included the fact in the passage. What role does it play in the author's overall point?

In the second paragraph, the author talks about a "long-standing puzzle" : there is "missing" baryonic mass in the universe. We know about this missing mass because of a gap between two different ways to measure baryonic mass -- helium and luminosity.

So, the author mentions the fact that baryons are the source of stars’ luminosity to connect these dots. That's a great fit with (C):
 
(C) is the correct answer to question 5.

Here's (E):
 
Again, when the author mentions the "fact," he/she is primarily trying to show how luminosity plays into the puzzle of the missing mass. He/she is not concerned with comparing the masses of different types of galaxies.

Additionally, the author would not agree that dim galaxies have fewer baryons than bright galaxies do -- in fact, we know from the first paragraph that they have the " same  approximate number of stars as a common type of conventional galaxy." It's just that dim galaxies are more spread out, making it tough to detect the luminosity of those galaxies. In other words, the luminosity measure was off because it failed to detect the baryons in the dim galaxies.

Eliminate (E) for question 5.

I hope that helps!

Sajjad1994 · Answer

Official Explanation

7. The primary purpose of the passage is to

Difficulty Level:   600

Explanation

Main idea

This question requires understanding, in broad terms, the purpose of the passage as a whole. The first paragraph describes a phenomenon: the discovery of dim galaxies and some of their general attributes. The second paragraph describes how this discovery may help astronomers to solve a long-standing puzzle about the baryonic mass of the universe.

A.  Correct.   The passage describes the phenomenon of dim galaxies and describes their significance in solving the long-standing puzzle of the missing baryonic mass in the universe.

B.  Although the passage discusses the puzzling difference between the two estimates of baryonic mass, this option does not account for the broader topic of dim galaxies.

C.  While the passage identifies the newly discovered phenomenon of dim galaxies, it does not offer a significant explanation for these galaxies’ origins.

D.  Although the passage compares dim and conventional galaxies in the first paragraph, this option does not account for the important detail that dim galaxies may help solve a long-standing puzzle.

E.  The discovery of dim galaxies discussed in the passage is not said to be inconsistent with any existing scientific theory.

Answer: A

Anonymous · Answer

In the second question ( It can be inferred from the passage that which of the following is an accurate physical description of typical low-surface-brightness galaxies?)  I chose (A) since in the first paragraph says ' they have far fewer stars per unit volume than do conventional galaxies ' and in combination with the fact that they are larger than conventional galaxies I inferred that they have fewer stars in total (for instance 10 stars per unit volume instead of 15, so they have 5 few per unit volume).

Where am I mistaken??  GMATCoachBen   GMATNinja   avigutman   chetan2u

Thanks in advance

In addition to conventional galaxies, the universe contains very dim

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In addition to conventional galaxies, the universe contains very dim

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