What evidence in the story suggests that Brown’s journey into the forest represents a journey into his own heart in Young Goodman Brown?

As is characteristic of Nathaniel Hawthorne, there is an
ambiguity to his story, "Young Goodman Brown" that Hawthorne himself pronounces after
Brown's experience in the forest: 

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Had Goodman Brown fallen asleep in the forest and
only dreamed a wild dream of a
witch-meeting?

Certainly,
there are indications that Brown indeed has a dream in which his subconscious expresses
doubt about his faith:

• When he parts from Faith,
  Goodman remarks to himself, "She talks of dreams, too," suggesting that his
  "experience" is truly a dream.


• As he travels, Goodman's
  perceptions are indistinct.  For instance, he feels that
  

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with lonely footsteps he may yet be passing
through an unseen
multitude.

• Goodman
  imagines apparitions, "a devilish Indian behind every tree." As it is supposedly dusk in
  the forest, Goodman cannot clearly see as he embarks on his journey.  Further in his
  dream, 

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owing doubtless to the depth of the
gloom at that particular spot, neither the travellers nor their steeds were
visible.

• Goodman
  Brown, perhaps, perceives his subconscious self in the old traveler who is in "the same
  rank as Goodman Brown, and bear[s] a considerable resemblance to
  him...."


• When Goodman watches the staff of the traveler
  wriggle like a snake, he thinks this must be "an ocular deception," certainly a
  dreamlike experience.  Brown also hears voices talk "so strangely in the empty
  air,"

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Aloft in the air, as if from the depths of the
cloud came a confused and doubtful sound of
voices.

• Goodman
  expresses a desire to turn back, but cannot seem to be able to do so, a recurring
  experience of dreams.


• Goodman goes farther than his
  ancestors and exclaims, "Too far! too far!" yet he continues as though the subconscious
  controls him. 


• As Goodman continues with the traveler,
  the elder's "arguments seemed rather to spring up in the bosom of his auditor than to be
  suggested by himself"--another indication of subconscious
  activity.


• Frequently, there is a blurring of what is real
  and what is not:

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Then came a stronger swell of those familiar
tones, hear daily...,but never until now from a cloud of night.  There was one voice of
a young woman uttering lamentations, yet with an uncertain
sorrow....

Either the sudden gleams of light flashing over
the obscure field bedazzled Goodman Brown, or he recognized a score of the church
members...[he] beheld a pink
ribbon

• Several
  magical things occur:  When the traveler laughs, his staff "wriggles in sympathy," Goody
  Cloyse flies on the staff of the traveler, and when he picks another fresh stick, the
  leaves on this stick immediately wither and die.  Also, one of the riders in the sky
  stops to pluck a switch. Even Goodman himself seems to fly along the forest
  path...

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leaving him in the heart of the dark wilderness,
still rushing onward with the instinct that guides mortal man to evil....But he was
himself the chief horror of the scene, and shrank not from its other horrors.....On he
flew among the black
pines....

• Hawthorne
  again hints at the subconscious of Brown, thus suggesting a dreamlike state in which
  Brown's doubt about his own goodness are
  expressed,

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The fiend in his own shape is less hideous than
when he rages in the breast of
man.

In the end, Young
Goodman Brown feels "a loathful brotherhood by the sympathy of all that was wicked in
his heart."  This statement of Hawthorne points to powerful role of the subconscious in
his narrative, that part of Goodman Brown expressed in his dream
journey.