"... on one of the golden worlds our long-range sensors have identified in Andromeda's Helius cluste

Come on we know the real answer to that:

I think what sageoflife meant is that, right now - or rather, in 2185 - we can point our ultra-mega-telescopes - that are, nonetheless, limited to lightspeed - at Andromeda, and see what was there five million years ago. As five million years is a relatively short stretch of time in astronomical terms, there's a very good chance that any habitable planets that were there five million years ago will still be there now--y'know, once you've accounted for cosmic drift and such. Earth, for example, has been able to support complex land-based life for at least the last 363 million years, and will only cease to be habitable somewhere between 1.75 and 3.25 billion years from now. 

We won't know exactly what we're getting into - that's the point of the game, after all; we're explorers - but the chances of every 'Golden World' candidate - assuming we chart more than a couple - having become uninhabitable in that five-million year window should be infinitesimally small. 

Of course, I'm on record as not really being overly fussed about whatever lore changes they want to make - I've been enjoying science fiction for far too long to allow inconsistencies to bother me that much - so I wouldn't actually care if they'd somehow invented Star Trek-style megasensors that worked beyond the speed of light - say, they took the same principle applied to the non-entangled FTL communication signals and expanded on it, for the appropriate technobabble - but I actually don't think that'd be necessary for this particular aspect of the Andromeda story.

Oh that video screams propaganda. 

Which I bet it will be a plot point. 

I don't see how that's so unbelievable.  The reapers started their invasion after the arks left, so any quantum entanglement communicators in the Milky Way could easily have been destroyed then.

I'm wait to see more, but it doesn't seem to be a good start or explanation for the lore jump. Though I already made my peace with it.
I just don't get why they wouldn't try to make the jump as lore friendly as possibly (assuming it isn't as it happens right now).

What they are detecting is what happened 2.5 million years ago. So when you get there you might find things are a bit different than what you thought you might find. As far as I am aware Bioware have not claimed anything to the contrary.

From what i have been seeing, more like f*** the lore...

You're right, I was thinking of RADAR and LADAR which are the two main forms of sensors used by ME ships. Even so, the inverse square law makes such detection methods impossible even at gamma ray burst intensities.

With that said:

The light from the Andromeda is not enough to reveal planets. Trying to look for habitable planets in Andromeda is like looking for bacteria-inhabited grains of sand on a beach at night in moonlight while standing on the moon.

Not 5 million years. That's completely the wrong number. It's not like we're firing a laser at Andromeda and waiting for the light to bounce off any planets and back to us. You have no reason to double the 2.5 million years that it takes light to traverse the distance.

Andromeda is already emitting light, it's just light that is 2.5 million years old by the time it gets to us. So there isn't a waiting problem, and 5 million years shouldn't enter into the equation at all. The Andromeda Initiative is presumably just using some kind of super-telescope to examine the light already coming from the Andromeda galaxy.

As an example age of a planet, Earth is about 4.5 billion years old. That's 3 orders of magnitude beyond the length of time that it takes light to travel from Andromeda to us. So yes, any planets that the Andromeda Initiative is looking at will actually be 2.5 million years older than they appear, but they will almost certainly still be there.

As it happens, in the real world, we may have already detected a planet in the Andromeda galaxy. It's a somewhat controversial result, but here: exoplanet.eu/catalog/pa-99-n2_b/ Note the insanely large distance of 670,000 parsecs, and the fact that "star in galaxy M31" is listed under remarks - M31 means Andromeda.

Detecting whether or not a planet is a garden world is trickier, but it has been done with closer exoplanets. It mostly involves looking for spectral lines from light passing through the planet's atmosphere. That said, the result I've listed above, if confirmed, is far too big to be Earth-like. It's more than 6 times the size of Jupiter. But at the end of the day, spotting exoplanets is mostly a question of telescope resolution, and being able to spot a star 'wobbling'. Our real world tech isn't really at the level where we can reliably detect exoplanets in Andromeda - but Mass Effect technology might be there.

As for the in-universe question of why can't we see Reapers if we can see planets in Andromeda - Reapers are in Dark Space, are small and dark themselves (compared to planets), and probably aren't orbiting a convenient star that we could look at for wobbles. They basically don't put out enough light to be spotted. We only spot planets by looking at a planet's host star and noticing it wobbling.

/thread?

Very much this.

BTW, it's only 2.5 million years in the past, worst case 2.7 million if the cluster is on the far edge of Andromeda. Not sure why Arcian made it 5 million. If it was a "golden world" 2.5 million years ago, chances are good it still is. So what's the beef?

If you want to complain about something, complain about how they resolved an open cluster in the galactic disk of Andromeda. It's hard to tell which stars are close together and in the cluster, or not in the cluster and nearer but dim, or not in the cluster and farther and bright. It all looks the same from here. And it has to be an open cluster, because globular clusters usually orbit galaxies as satellites. Not to mention a bunch of other problems, like low metallicity of the stars.

EDIT: Oh crap, and what jasonshepardn7 said -- nevermind not being able to resolve a planet and it's characteristics, you can't even detect that stars are in a cluster.